Fill area between curves

Using the fill_between parameter of the pygmt.Figure.plot method it is possible to fill the area between two curves y1 and y2. Different fills (colors or patterns) can be used for the areas y1 > y2 and y1 < y2. Optionally, the curves can be drawn. To plot an anomaly along a track use pygmt.Figure.grdlandmask and see the gallery example Wiggle along tracks.

import numpy as np
import pandas as pd
import pygmt as gmt

# Generate some test data and create a pandas DataFrame
x = np.arange(-10, 10.2, 0.1)
y1 = np.sin(x * 3)
y2 = np.sin(x / 2)

data_df = pd.DataFrame({"x": x, "y1": y1, "y2": y2})

Fill the areas between the two curves using the fill_between parameter. Use the fill parameter and the modifier +g for fill_between to set different fills for areas with y1 > y2 and y2 < y1, respectively. Use the label parameter and the modifier +l for fill_between to set the corresponding legend entries.

fig = gmt.Figure()
fig.basemap(region=[-10, 10, -5, 5], projection="X15c/5c", frame=True)

fig.plot(
    data=data_df,
    fill="orange",
    label="short > long",
    fill_between="c+gsteelblue+lshort < long",
)

fig.legend()

fig.show()

Additionally to filling the areas, we can draw the curves. Use the pen parameter and the modifier +p for fill_between to set different lines for the two curves y1 and y2, respectively.

fig = gmt.Figure()
fig.basemap(region=[-10, 10, -5, 5], projection="X15c/5c", frame=True)

fig.plot(
    data=data_df,
    fill="p8",
    pen="1p,black,solid",
    fill_between="c+gp17+p1p,black,dashed",
)

fig.show()

To compare a curve y1 to a horizontal line, append +y to fill_between and give the desired y-level.

fig = gmt.Figure()
fig.basemap(region=[-10, 10, -5, 5], projection="X15c/5c", frame=True)

fig.plot(
    data=data_df[["x", "y1"]],
    fill="p8",
    pen="1p,black,solid",
    # Define a horizontal line at y=0.42
    fill_between="c+gp17+p1p,black,dashed+y0.42",
)

fig.show()