Style Symbol Cartograms¶

This guide shows how to use data-driven styling when plotting a symbol cartogram. The plot() method maps any data column to fill color, opacity, edge color, line width, hatching, or symbol shape — each independently.

The workflow separates layout (expensive, run once) from styling (fast, experiment freely):

1. create_layout() — computes symbol positions; stores them in an immutable LayoutResult
2. layout_result.style() — applies symbol shapes
3. cartogram.plot() — maps data to color, opacity, hatching, labels

In [1]:

import matplotlib.pyplot as plt

import carto_flow.symbol_cartogram as sym
from carto_flow.data import load_us_census
from carto_flow.symbol_cartogram.styling import Styling

gdf = load_us_census(poverty=True)
gdf.head()

import matplotlib.pyplot as plt import carto_flow.symbol_cartogram as sym from carto_flow.data import load_us_census from carto_flow.symbol_cartogram.styling import Styling gdf = load_us_census(poverty=True) gdf.head()

Out[1]:

	Population	Total Poverty	Below Poverty Level	Above Poverty Level	STATE	State Abbreviation	State Name	geometry	Region	Division	Population (Millions)	Population Density	Below Poverty Level %	Above Poverty Level %
0	12794885	11697147	1354558	10342589	42	PA	Pennsylvania	POLYGON ((1200847.835 331701.964, 1203743.68 3...	Northeast	Middle Atlantic	12.794885	109.040186	0.115802	0.884198
1	39346023	36223360	4451904	31771456	06	CA	California	MULTIPOLYGON (((-1979058.42 -528956.745, -1975...	West	Pacific	39.346023	96.001918	0.122901	0.877099
2	1807426	1662817	276138	1386679	54	WV	West Virginia	POLYGON ((1100235.176 -108059.819, 1102295.003...	South	South Atlantic	1.807426	28.802429	0.166066	0.833934
3	3151239	2856137	255847	2600290	49	UT	Utah	POLYGON ((-1396894.253 368735.242, -1392505.21...	West	Mountain	3.151239	14.331142	0.089578	0.910422
4	19514849	17890963	2360844	15530119	36	NY	New York	MULTIPOLYGON (((1253147.537 375607.401, 125587...	Northeast	Middle Atlantic	19.514849	153.727217	0.131957	0.868043

Compute the layout once¶

create_layout() runs the circle packing simulation and returns a LayoutResult. All styling variations below reuse this single result — no re-simulation needed.

In [2]:

layout_result = sym.create_layout(gdf, "Population", layout="topology", size_normalization="total", show_progress=False)
cartogram = layout_result.style()

layout_result = sym.create_layout(gdf, "Population", layout="topology", size_normalization="total", show_progress=False) cartogram = layout_result.style()

Color by a numeric column¶

Pass a column name to facecolor together with a colormap. vmin and vmax pin the color range.

In [3]:

fig, ax = plt.subplots(figsize=(12, 7))

plot = cartogram.plot(
    ax=ax,
    # pass the original DataFrame
    # to have access to the data columns
    source_gdf=gdf,
    # numeric-column-driven face color
    facecolor="Below Poverty Level %",
    cmap="YlOrRd",
    # show colorbar
    legend=True,
    legend_kwds={"shrink": 0.6},
)

ax.set_title("Symbols sized by Population and colored by Poverty Level")
ax.axis("off")

plt.tight_layout()

fig, ax = plt.subplots(figsize=(12, 7)) plot = cartogram.plot( ax=ax, # pass the original DataFrame # to have access to the data columns source_gdf=gdf, # numeric-column-driven face color facecolor="Below Poverty Level %", cmap="YlOrRd", # show colorbar legend=True, legend_kwds={"shrink": 0.6}, ) ax.set_title("Symbols sized by Population and colored by Poverty Level") ax.axis("off") plt.tight_layout()

Color by a categorical column¶

Pass a categorical column name and a color_map dict that maps each value to a color.

In [4]:

region_colors = {
    "Northeast": "#4e79a7",
    "South": "#f28e2b",
    "Midwest": "#59a14f",
    "West": "#e15759",
}

fig, ax = plt.subplots(figsize=(12, 7))
cartogram.plot(
    ax=ax,
    # pass the original DataFrame
    # to have access to the data columns
    source_gdf=gdf,
    # categorical-column-driven facecolor
    facecolor="Region",
    # specify custom category to color mapping
    cmap=region_colors,
    # show legend
    legend=True,
    legend_kwds={"loc": "upper left", "framealpha": 0.8},
)
ax.set_title("Colored by Census Region")
ax.axis("off")
plt.tight_layout()

region_colors = { "Northeast": "#4e79a7", "South": "#f28e2b", "Midwest": "#59a14f", "West": "#e15759", } fig, ax = plt.subplots(figsize=(12, 7)) cartogram.plot( ax=ax, # pass the original DataFrame # to have access to the data columns source_gdf=gdf, # categorical-column-driven facecolor facecolor="Region", # specify custom category to color mapping cmap=region_colors, # show legend legend=True, legend_kwds={"loc": "upper left", "framealpha": 0.8}, ) ax.set_title("Colored by Census Region") ax.axis("off") plt.tight_layout()

Different symbol shapes per category¶

Build a Styling object and call set_symbol() with a boolean mask for each category. Shape assignment is independent of plot() color options — both can vary at the same time.

In [5]:

region_shapes = {
    "Northeast": "square",
    "South": "circle",
    "Midwest": "hexagon",
    "West": "diamond",
}

# style symbol shapes
styling = Styling(scale=0.85)
for region, shape in region_shapes.items():
    mask = (gdf["Region"] == region).values
    styling = styling.set_symbol(shape, mask=mask)

# note that cartograms can be restyled
cartogram_shapes = cartogram.restyle(styling=styling)

fig, ax = plt.subplots(figsize=(12, 7))
cartogram_shapes.plot(
    ax=ax,
    # pass the original DataFrame
    # to have access to the data columns
    source_gdf=gdf,
    facecolor="steelblue",
)

ax.set_title("Shape encodes Census region")
ax.axis("off")
plt.tight_layout()

region_shapes = { "Northeast": "square", "South": "circle", "Midwest": "hexagon", "West": "diamond", } # style symbol shapes styling = Styling(scale=0.85) for region, shape in region_shapes.items(): mask = (gdf["Region"] == region).values styling = styling.set_symbol(shape, mask=mask) # note that cartograms can be restyled cartogram_shapes = cartogram.restyle(styling=styling) fig, ax = plt.subplots(figsize=(12, 7)) cartogram_shapes.plot( ax=ax, # pass the original DataFrame # to have access to the data columns source_gdf=gdf, facecolor="steelblue", ) ax.set_title("Shape encodes Census region") ax.axis("off") plt.tight_layout()

Data-driven opacity¶

Pass a column name to alpha. alpha_range sets the min and max opacity.

In [6]:

fig, ax = plt.subplots(figsize=(12, 7))
cartogram.plot(
    ax=ax,
    # pass the original DataFrame
    # to have access to the data columns
    source_gdf=gdf,
    facecolor="steelblue",
    # numeric-column-driven alpha
    alpha="Below Poverty Level %",
    alpha_range=(0.2, 1.0),
    alpha_legend=True,
)
ax.set_title("Opacity encodes Poverty Level")
ax.axis("off")
plt.tight_layout()

fig, ax = plt.subplots(figsize=(12, 7)) cartogram.plot( ax=ax, # pass the original DataFrame # to have access to the data columns source_gdf=gdf, facecolor="steelblue", # numeric-column-driven alpha alpha="Below Poverty Level %", alpha_range=(0.2, 1.0), alpha_legend=True, ) ax.set_title("Opacity encodes Poverty Level") ax.axis("off") plt.tight_layout()

Data-driven edge color and line width¶

edgecolor accepts a column name plus an optional edge_cmap. linewidth accepts a column name plus linewidth_range.

In [7]:

fig, ax = plt.subplots(figsize=(12, 7))
cartogram.plot(
    ax=ax,
    # pass the original DataFrame
    # to have access to the data columns
    source_gdf=gdf,
    facecolor="lightgray",
    # category-column-driven edge color
    # with custom color mapping
    edgecolor="Region",
    edge_cmap=region_colors,
    # numeric-column-driven line width
    linewidth="Below Poverty Level %",
    linewidth_range=(0.3, 3.0),
    # legends
    edge_legend=True,
    edge_legend_kwds={"loc": "upper center"},
    linewidth_legend=True,
    linewidth_legend_kwds={"loc": "upper left"},
)
ax.set_title("Edge color encodes Region and edge width encodes Poverty Level")
ax.axis("off")
plt.tight_layout()

fig, ax = plt.subplots(figsize=(12, 7)) cartogram.plot( ax=ax, # pass the original DataFrame # to have access to the data columns source_gdf=gdf, facecolor="lightgray", # category-column-driven edge color # with custom color mapping edgecolor="Region", edge_cmap=region_colors, # numeric-column-driven line width linewidth="Below Poverty Level %", linewidth_range=(0.3, 3.0), # legends edge_legend=True, edge_legend_kwds={"loc": "upper center"}, linewidth_legend=True, linewidth_legend_kwds={"loc": "upper left"}, ) ax.set_title("Edge color encodes Region and edge width encodes Poverty Level") ax.axis("off") plt.tight_layout()

Hatching¶

hatch accepts a column name and hatch_map maps each category to a matplotlib hatch string. hatch_legend=True adds a legend entry for each hatch pattern.

In [8]:

hatch_map = {
    "Northeast": "////",
    "South": "....",
    "Midwest": "xxxx",
    "West": "----",
}

fig, ax = plt.subplots(figsize=(12, 7))
cartogram.plot(
    ax=ax,
    # pass the original DataFrame
    # to have access to the data columns
    source_gdf=gdf,
    facecolor="Below Poverty Level %",
    legend_kwds={"shrink": 0.7},
    hatch="Region",
    hatch_map=hatch_map,
    hatch_legend=True,
    hatch_legend_kwds={"title": "Region", "loc": "upper left"},
    # set edge color for hatching
    edgecolor="white",
)

ax.set_title("Color encoded Poverty Level and hatching encodes region")
ax.axis("off")

plt.tight_layout()

hatch_map = { "Northeast": "////", "South": "....", "Midwest": "xxxx", "West": "----", } fig, ax = plt.subplots(figsize=(12, 7)) cartogram.plot( ax=ax, # pass the original DataFrame # to have access to the data columns source_gdf=gdf, facecolor="Below Poverty Level %", legend_kwds={"shrink": 0.7}, hatch="Region", hatch_map=hatch_map, hatch_legend=True, hatch_legend_kwds={"title": "Region", "loc": "upper left"}, # set edge color for hatching edgecolor="white", ) ax.set_title("Color encoded Poverty Level and hatching encodes region") ax.axis("off") plt.tight_layout()

Labels¶

label adds text from a column. label_color and label_fontsize can each be a fixed value or a data column name.

In [9]:

fig, ax = plt.subplots(figsize=(14, 8))
cartogram.plot(
    ax=ax,
    source_gdf=gdf,
    facecolor="Region",
    alpha=0.5,
    # color_map=region_colors,
    edgecolor="white",
    linewidth=0.3,
    label="State Abbreviation",
    label_color="Below Poverty Level %",
    label_cmap="hot",
    label_fontsize=8,
    label_kwargs={"fontweight": "bold"},
    label_legend=True,
)
ax.set_title("Abbreviated State name labels with Poverty Level encoded text color")
ax.axis("off")
plt.tight_layout()

fig, ax = plt.subplots(figsize=(14, 8)) cartogram.plot( ax=ax, source_gdf=gdf, facecolor="Region", alpha=0.5, # color_map=region_colors, edgecolor="white", linewidth=0.3, label="State Abbreviation", label_color="Below Poverty Level %", label_cmap="hot", label_fontsize=8, label_kwargs={"fontweight": "bold"}, label_legend=True, ) ax.set_title("Abbreviated State name labels with Poverty Level encoded text color") ax.axis("off") plt.tight_layout()