Use smaller figure dimensions in the "Plot lines" tutorial

[seisman]

Examples in the Plot lines tutorial all use the projection="X25c/20c" argument, which sets a 25cm X 20cm figure.

The figure dimensions (25cm X 20cm) is too large, making the annotations too small to see.

Additionally, figures with large dimensions also have larger sizes, and are slower to generate. We should update the examples to use smaller figure dimensions.

In short, we need to change projection="X25c/20c" to projection="X15c/10c" for all examples.

The source code of the tutorial is at https://github.com/GenericMappingTools/pygmt/blob/master/examples/tutorials/lines.py.

[xdshivani]

Assign this to me

[shahid-0]

I am ready to do this. assign me