Auto-generated legend doesn't work with "custom" (k) symbols

[ibrewster]

Description of the problem
When calling Figure.plot, you can specify a custom symbol by specifying the symbol with a k followed by the custom symbol name/file, such as kvolcano/16p. However, doing so breaks the auto-generated legend, failing with the following output:

legend [INFORMATION]: Processing input text table data
legend [INFORMATION]: Processing hidden legend specification file /Users/israel/.gmt/sessions/gmt_session.21374/gmt.1.legend
legend [INFORMATION]: Reading Data Table from File /Users/israel/.gmt/sessions/gmt_session.21374/gmt.1.legend
legend [INFORMATION]: Central meridian not given, default to -179.678
legend [INFORMATION]: Map scale is 9.45374 km per cm or 1:945374.
legend [INFORMATION]: Legend height not given, using estimated height of 1.40367 cm.
plot [INFORMATION]: Processing input table data
plot [INFORMATION]: Map scale is 0.001 km per inch or 1:39.3701.
plot [INFORMATION]: Reading Data Table from memory reference supplied by pointer
plot [ERROR]: Could not find either custom symbol or EPS macro

Looking at the temporary file generated, we find the following content:

# Auto-generated legend information file
# LEGEND_JUSTIFICATION: RT
# LEGEND_SCALING: 1
# LEGEND_FRAME:  1p white 0.2c
S - k 0.222222i red - - Plot1
S - k 0.222222i green - - Plot2

In which we see that it put the "k" for the symbol into the file, but failed to specify the custom symbol to use. In fact, if we modify this autogenerated file to add the symbol name again, like so:

# Auto-generated legend information file
# LEGEND_JUSTIFICATION: RT
# LEGEND_SCALING: 1
# LEGEND_FRAME:  1p white 0.2c
S - kvolcano 0.222222i red - - Plot1
S - kvolcano 0.222222i green - - Plot2

and then remove the label entries from our plots and feed the modified file in as the first parameter to legend (if we don't remove the label= entries from the plot command it always tries to use them, even with a file specified), we get the desired result.

Full code that generated the error

import pygmt

bounds = [-180.7568317475191,
          -178.59975221352386,
          51.37616035103776,
          52.375229777638786]

fig = pygmt.Figure()
fig.basemap(projection="M10i", region=bounds, frame=('WeSn', 'afg'))

sta_x = -179
fig.plot(x = [sta_x, ], y = [52, ], style = 'kvolcano/16p', color = 'red')
sta_x = 179.75
fig.plot(x = [sta_x, ], y = [52, ], style = 'kvolcano/16p', color = 'green')

pos = 'JTR+jTR+o0.2c+l1.5'

#This fails with the above error
fig.legend(position = pos, box = "+gwhite+p1p", verbose = True)

fig.show(method = "external")

NOTE: I have not tried this on straight GMT, only PyGMT.

System information

Please paste the output of python -c "import pygmt; pygmt.show_versions()":

import pygmt; pygmt.show_versions()
PyGMT information:
  version: v0.5.0
System information:
  python: 3.9.5 (v3.9.5:0a7dcbdb13, May  3 2021, 13:17:02)  [Clang 6.0 (clang-600.0.57)]
  executable: /Applications/Wing Pro.app/Contents/Resources/wingdb
  machine: macOS-10.15.7-x86_64-i386-64bit
Dependency information:
  numpy: 1.21.2
  pandas: 1.3.3
  xarray: 0.19.0
  netCDF4: 1.5.7
  packaging: 21.0
  ghostscript: 9.55.0
  gmt: 6.3.0
GMT library information:
  binary dir: /Library/Frameworks/Python.framework/Versions/3.9/Resources/Python.app/Contents/MacOS
  cores: 8
  grid layout: rows
  library path: /usr/local/Cellar/gmt/6.3.0_2/lib/libgmt.dylib
  padding: 2
  plugin dir: /usr/local/Cellar/gmt/6.3.0_2/lib/gmt/plugins
  share dir: /usr/local/Cellar/gmt/6.3.0_2/share/gmt
  version: 6.3.0

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[maxrjones]

GMT does not yet support auto-generated legend items for custom symbols. This issue is being tracked upstream by GenericMappingTools/gmt#1973.