Add gallery example for 3-D bar plot (#4315)

Author: yvonnefroehlich

examples/gallery/3d_plots/3d_bar.py

@@ -0,0 +1,69 @@
+"""
+3-D bar plot
+============
+
+A 3-D bar plot can be created from any collection of three-dimensional tabular data. The
+data points can lie on a regular grid or be irregularly scattered. A special case is
+creating such a 3-D bar plot based on a grid. This can be done in two steps:
+
+1. Converting the grid into a table via :func:`pygmt.grd2xyz`, with columns "x", "y",
+   and "z" for longitude, latitude, and the quantity displayed by the grid,
+   respectively.
+2. Plotting this table as bars in 3-D using :meth:`pygmt.Figure.plot3d`.
+
+The bars can be outlined, and the fill can be one color or based on a quantity using a
+colormap. For the latter, a fourth column needs to be added containing the values of the
+quantity for the color-coding.
+"""
+
+# %%
+import pygmt
+from pygmt.params import Position
+
+# Define a study area around northern Japan with large elevation changes
+region = [141, 147, 36, 43]
+
+# Download a grid for the Earth relief with a resolution of 10 arc-minutes
+grid = pygmt.datasets.load_earth_relief(resolution="10m", region=region)
+
+# Convert the grid into a pandas DataFrame, with columns for longitude ("x"), latitude
+# ("y") and elevation ("z")
+grd_df = pygmt.grd2xyz(grid=grid)
+zmin = grd_df["z"].min() - 50
+zmax = grd_df["z"].max() + 50
+
+# Add a fourth column "color" for the quantity used for the color-coding of the bars,
+# here we use the elevation ("z")
+grd_df["color"] = grd_df["z"]
+
+# Create a 3-D bar plot with color-coding
+fig = pygmt.Figure()
+
+fig.basemap(
+    region=[*region, zmin, zmax],
+    projection="M10c",
+    zsize="8c",
+    frame=["WSneZ", "xaf", "yag", "za1000f500+lElevation / m"],
+    perspective=(195, 30),
+)
+
+pygmt.makecpt(cmap="SCM/oleron", series=(zmin, zmax))
+fig.plot3d(
+    data=grd_df,
+    # Use "o" to plot bars and give the desired size
+    # The base of the bars is set via "+b"
+    style=f"o0.34c+b{zmin}",
+    cmap=True,
+    pen="0.01p,gray30",
+    perspective=True,
+)
+fig.colorbar(
+    frame=["xa1000f500+lElevation", "y+lm"],
+    position=Position("TR", cstype="inside", offset=1.4),
+    orientation="vertical",
+    length=7,
+    move_text="label",
+    label_as_column=True,
+)
+
+fig.show()