Allow passing an array as symbols for plot

[core-man]

Description of proposed changes

We can only plot different symbols via the data parameter in the plot method. This PR allows passing an array as symbols via the new symbol parameter. Meanwhile, we have to set style="0.2c", or set 1d-array sizes using style=True and sizes=sizes. Therefore, we now have four ways to set symbols and sizes, e.g.,

• constant symbol and size: style="c0.5c"
• constant symbol and different sizes: style="cc" + sizes=sizes
• different symbols and constant size: style="0.5c" + symbol=symbol
• different symbols and sizes: style=True + sizes=sizes + symbol=symbol

Two tests are added:

• test_plot_fail_1d_array_using_data: Raise GMTInvalidInput if symbol is used with data parameter. Actually, I rename test_plot_fail_color_size_intensity to be this test function since we should raise such an error for 1d-array color, size, intensity and symbol parameters if they are used with the data parameter. (originally posted in The transparency parameter of plot/plot3d methods can be 1d array #1098 (comment))
• test_plot_symbols: test 1d-array symbol (all are "c") with style="0.5c".

Related #1076

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[core-man]

1c48b39: If the symbol parameter is used, we will automatically set style to be True if it is not set by the user.

Therefore, now we can plot different symbols and sizes like this: symbol=symbol, sizes=sizes. Note that the sizes parameter only accepts int and float (e.g., 1, 0.5) and does not accept string (e.g., 0.5c), so the unit can have to be set by the PROJ_LENGTH_UNIT using the config method if we don't want to use the default cm. (Also posted in #1076 (comment))

[core-man]

I added isinstance(kwargs["S"], str) in case that the style parameter is True, False, None, int, or float.

[seisman]

@core-man I think the co-existence of style and symbol parameters will lead to more confusion. We should discuss what we would like to use about the plot() and plot3d() functions.

[core-man]

@core-man I think the co-existence of style and symbol parameters will lead to more confusion. We should discuss what we would like to use about the plot() and plot3d() functions.

Actually, I'd like to fully avoid using the style parameter, and use a more Pythonic way, e.g., sizes, symbol, direction. But now I don't know if how much work will have to be done before the style parameter is avoided since plot/plot3d still have many unimplemented and complicated features/options.

We could remove the style parameter now and directly use symbol instead (see your suggestion in #1072 (comment)? What do you think? This is also related to Backwards compatibility and deprecation policy (#1120).

[seisman]

I believe we need more discussions about what we would like to see about the plot() function.
I'll convert this PR to draft.

[weiji14]

At the moment, this test is checking that plotting all symbols as circles (c) work. A better test would be to test a mix of symbols such as circles (c), triangles (t), squares (s), etc (like your example in #1076 (comment)).

[weiji14]

@core-man, ok if one of us picks this PR up again? There's been quite a few changes to the plot function including renaming of aliases (see https://github.com/GenericMappingTools/pygmt/commits/v0.9.0/pygmt/src/plot.py), so we might need to resolve a few conflicts first.