Add an advanced tutorial for creating legends

examples/tutorials/advanced/legends.py

@@ -0,0 +1,138 @@
+"""
+Creating legends
+================
+
+The :meth:`pygmt.Figure.legend` method creates legends, whereby auto-legends as
+well as manually created legends are supported.
+
+Users can adjust the position of the legend and add a box around the legend.
+"""
+
+# %%
+import io
+
+import pygmt
+
+# %%
+# Create an auto-legend
+# ---------------------
+#
+# For auto-legends, the ``label`` parameter of :meth:`pygmt.Figure.plot` has to
+# be specified to state the desired text in the legend entry (white spaces are
+# allowed). Optionally, to adjust the legend, users can append different modifiers
+# to the string passed to ``label`. A list of all available modifiers can be
+# found at :gmt-docs:`gmt.html#l-full`. To create a multiple-column legend **+N**
+# is used with the desired number of columns; see also gallery example
+# https://www.pygmt.org/dev/gallery/embellishments/legend.html.
+
+fig = pygmt.Figure()
+fig.basemap(region=[-5, 5, -5, 5], projection="X5c", frame=True)
+
+fig.plot(x=0, y=-3, style="c0.25c", fill="orange", label="orange circle")
+fig.legend()
+
+fig.show()
+
+
+# %%
+# Adjust the position
+# -------------------
+# Use the ``position`` parameter to adjust the position of the legend.
+# j within, J outside of bounding box
+# also adjust ``width`` via **+w** modifier
+# The default of ``box`` is changed, i.e. no box plotted anymore.
+
+fig = pygmt.Figure()
+fig.basemap(region=[-5, 5, -5, 5], projection="X5c", frame=True)
+
+fig.plot(x=0, y=-3, style="c0.25c", fill="orange", label="orange circle")
+fig.legend(position="jMC")
+
+fig.show()
+
+
+# %%
+# Add a box
+# ---------
+# ``box`` parameter, Default: 1-point thick, black, solid outline with white fill.
+# The default of ``position`` is preserved.
+# **+p** for outline
+# **+g** for fill
+
+fig = pygmt.Figure()
+fig.basemap(region=[-5, 5, -5, 5], projection="X5c", frame=True)
+
+fig.plot(x=0, y=-3, style="c0.25c", fill="orange", label="orange circle")
+fig.legend(position="jMC", box=True)
+
+fig.shift_origin(xshift="w+1c")
+fig.basemap(region=[-5, 5, -5, 5], projection="X5c", frame=True)
+
+fig.plot(x=0, y=-3, style="c0.25c", fill="orange", label="orange circle")
+fig.legend(position="jMC", box="+p2p,cyan+gblue@70")
+
+fig.show()
+
+
+# %%
+# Create a manual legend
+# ----------------------
+#
+# For more complicated legends in GMT, users need to write an ASCII file with
+# instructions for the layout of the legend items. In PyGMT it is addionaly
+# possible to provide this information as an ``io.StringIO`` object. Both, the
+# ASCII file or the ``io.StringIO`` object are passed to the ``spec`` parameter
+# of :meth:`pygmt.Figure.legend`.
+#
+# The example below is orientated on the upstream GMT example at
+# https://docs.generic-mapping-tools.org/dev/legend.html#examples.
+
+
+# %%
+# First, we set up an ``io.StringIO`` object.
+
+spec_io = io.StringIO(
+    """
+G -0.1c
+H 24p,Times-Roman My Map Legend
+D 0.2c 1p
+N 2
+V 0 1p
+S 0.1c c 0.15c p300/12 0.25p 0.3c This circle is hachured
+S 0.1c e 0.15c yellow 0.25p 0.3c This ellipse is yellow
+S 0.1c w 0.15c green 0.25p 0.3c This wedge is green
+S 0.1c f 0.25c blue 0.25p 0.3c This is a fault
+S 0.1c - 0.15c - 0.25p,- 0.3c A contour
+S 0.1c v 0.25c magenta 0.5p 0.3c This is a vector
+S 0.1c i 0.15c cyan 0.25p 0.3c This triangle is boring
+D 0.2c 1p
+V 0 1p
+N 1
+M 5 5 600+u+f
+G 0.05c
+I @SOEST_block4.png 3i CT
+G 0.05c
+G 0.05c
+L 9p,Times-Roman R Smith et al., @%5%J. Geophys. Res., 99@%%, 2000
+G 0.1c
+T Let us just try some simple text that can go on a few lines.
+T There is no easy way to predetermine how many lines may be required
+T so we may have to adjust the height to get the right size box.
+"""
+)
+
+# %%
+# Now, we can add a legend based on this `io.StringIO` object. For
+# multi-columns legends, width (**+w**) has to be specified via a the
+# ``position`` parameter.
+
+fig = pygmt.Figure()
+# Note, that we are now using a Mercator projection
+fig.basemap(region=[-5, 5, -5, 5], projection="M10c", frame=True)
+
+# Pass the io.StringIO object to the spec parameter
+fig.legend(spec=spec_io, position="jMC+w9c", box="+p1p,gray50+ggray95")
+
+fig.show()
+
+# sphinx_gallery_thumbnail_number = 4