Transparency of colormap does not apply to color-coding for using Figure.grdimage

[yvonnefroehlich]

Description of the problem

It's possible to add transparency directly to a colormap while creating it via pygmt.makecpt.

When using this colormap for plotting data points with color-coding via Figure.plot, this transparency is considered, but when plotting a grid with color-coding via Figure.grdimage it is ignored. A workaround is specifying the transparency parameter (with the same value) again for Figure.grdimage.

(I) data points (pandas.DataFrame) via Figure.plot	(II) grid (Xarray.DataArray) via Figure.grdimage

Minimal Complete Verifiable Example

# (I) Plotting data points with color-coding
import pygmt

df = pygmt.datasets.load_sample_data(name="japan_quakes")

fig = pygmt.Figure()
fig.basemap(region=[135, 150, 35, 50], projection="M10c", frame=True)

pygmt.makecpt(cmap="viridis", series=[0, 600], transparency=50)
fig.plot(
    x=df.longitude,
    y=df.latitude,
    size=[0.5] * len(df),
    fill=df.depth_km,
    cmap=True,
    style="cc",
)
fig.colorbar(frame=True)

fig.show()

# -----------------------------------------------------------------
# (II) Plotting a grid with color-coding
import pygmt

grid = pygmt.datasets.load_earth_relief()

fig = pygmt.Figure()
fig.basemap(region=[-10, 10, -10, 10], projection="M10c", frame=True)

pygmt.makecpt(cmap="oleron", series=[-1000, 500], transparency=50)
fig.grdimage(grid=grid, cmap=True)
fig.colorbar(frame=True)

fig.show()

Full error message

Not applicable.

System information

import pygmt; pygmt.show_versions()
PyGMT information:
  version: v0.13.0
System information:
  python: 3.12.3 | packaged by conda-forge | (main, Apr 15 2024, 18:20:11) [MSC v.1938 64 bit (AMD64)]
  executable: C:\ProgramData\Anaconda3\envs\pygmt_env_dev_650\python.exe
  machine: Windows-10-10.0.19045-SP0
Dependency information:
  numpy: 1.26.4
  pandas: 2.2.2
  xarray: 2024.3.0
  netCDF4: 1.6.5
  packaging: 24.0
  contextily: 1.6.0
  geopandas: 0.14.3
  IPython: 8.22.2
  rioxarray: 0.15.4
  gdal: 3.8.2
  ghostscript: 10.03.0
GMT library information:
  version: 6.5.0
  padding: 2
  share dir: C:/Program Files (x86)/gmt6/share
  plugin dir: C:/ProgramData/Anaconda3/envs/pygmt_env_dev_650/Library/bin/gmt_plugins
  library path: C:/ProgramData/Anaconda3/envs/pygmt_env_dev_650/Library/bin/gmt.dll
  cores: 4
  grid layout: rows
  image layout: 
  binary version: 6.5.0