why the scatter plot can not be on the map created by cartopy

Question

plt.figure(figsize = (10,10),dpi =150)
ax = plt.axes(projection=ccrs.EuroPP())

ax.coastlines(resolution='50m')
ax.set_extent([-16., 5., 49., 61.])


plt.scatter([10.,15.],[52.5,55.],marker='o',s = 100,
            transform = ccrs.EuroPP(), color = "red")

ax.gridlines(draw_labels=True)
plt.show()

Why there is only a map, and the scatter plot cannot be on the map?

Answer 1

According to the source code https://scitools.org.uk/cartopy/docs/v0.15/_modules/cartopy/crs.html#EuroPP The plotting limits are set in the lines of code:-

@property
def x_limits(self):
    return (-1.4e6, 2e6)

@property
def y_limits(self):
    return (4e6, 7.9e6)

The values are in projection coordinates with meters unit. The scatter points that fall outside these limits will not be plotted.

Answer 2

The transform you specify for the scatter should be the projection of the data, not the map. It looks as if they are lat/lon values (epsg:4326)? (Technically they could of course be EuroPP coordinates, just really close to the origin)

Where do you expect those points to be more or less?

map_proj = ccrs.EuroPP()
data_proj = ccrs.PlateCarree()

fig, ax = plt.subplots(
    figsize=(10,10), dpi=76, 
    subplot_kw=dict(projection=map_proj),
)

ax.scatter(
    [10.,15.],[52.5,55.], marker='o', s=100, 
    color="r", transform=data_proj,
)

ax.coastlines(resolution='50m')
ax.set_extent([-5., 20., 49., 61.], crs=data_proj)
ax.gridlines(draw_labels=True)