feat: allow LinePlot to be drawn in steps

[larshei]

Changes

Many timeseries depict a "state" of some sort that was measured at specific points in time.

Simply drawing a line between the points of measurement often feels weird to me. I rather think "the value has been y from t1 to t2, then at t2 we got a new value".

So I added a style for line graphs that creates a LinePlot with steps:

As we no longer have two options (smooth yes/no) but three options, the interface needed to be changed:

The option is now called plot_type, with the options

• :direct
• :smooth
• :step

If plot_type is not provided, we fall back to the smoothed option.