Plot only the magnetic anomaly for Osborne mine (#24)

Adding a plot of the flight height takes double the time and doesn't
include any more useful information. Plot only the anomaly data instead.
Also avoids the PyGMT subplot code that can be a bit confusing.

doc/gallery_src/osborne-magnetic.py

@@ -36,56 +36,32 @@
 data
 
 ###############################################################################
-# Make two PyGMT maps with the data points colored by the total field magnetic
-# anomaly and also the observation height.
-
-region = [
-    data.longitude.min(),
-    data.longitude.max(),
-    data.latitude.min(),
-    data.latitude.max(),
-]
-
+# Make a PyGMT map with the data points colored by the total field magnetic
+# anomaly.
 fig = pygmt.Figure()
-with fig.subplot(
-    nrows=1,
-    ncols=2,
-    figsize=("30c", "20c"),
-    sharey="l",  # shared y-axis on the left side
-    frame="WSrt",
-):
-    with fig.set_panel(0):
-        fig.basemap(projection="M?", region=region, frame="af")
-        scale = 1500
-        pygmt.makecpt(cmap="polar+h", series=[-scale, scale], background=True)
-        fig.plot(
-            x=data.longitude,
-            y=data.latitude,
-            color=data.total_field_anomaly_nt,
-            style="c0.075c",
-            cmap=True,
-        )
-        fig.colorbar(
-            frame='af+l"total field magnetic anomaly [nT]"',
-            position="JBC+h+o0/1c+e",
-        )
-    with fig.set_panel(1):
-        fig.basemap(projection="M?", region=region, frame="af")
-        pygmt.makecpt(
-            cmap="viridis",
-            series=[data.height_orthometric_m.min(), data.height_orthometric_m.max()],
-        )
-        fig.plot(
-            x=data.longitude,
-            y=data.latitude,
-            color=data.height_orthometric_m,
-            style="c0.075c",
-            cmap=True,
-        )
-        fig.colorbar(
-            frame='af+l"observation height [m]"',
-            position="JBC+h+o0/1c",
-        )
+fig.basemap(
+    projection="M15c",
+    region=[
+        data.longitude.min(),
+        data.longitude.max(),
+        data.latitude.min(),
+        data.latitude.max(),
+    ],
+    frame="af",
+)
+scale = 1500
+pygmt.makecpt(cmap="polar+h", series=[-scale, scale], background=True)
+fig.plot(
+    x=data.longitude,
+    y=data.latitude,
+    color=data.total_field_anomaly_nt,
+    style="c0.075c",
+    cmap=True,
+)
+fig.colorbar(
+    frame='af+l"total field magnetic anomaly [nT]"',
+    position="JBC+h+o0/1c+e",
+)
 fig.show()
 
 ###############################################################################