Switch to horizontal orientation by default

[sethaxen]

I think a horizontal default orientation makes more sense than a vertical one. There are several reasons for this:

• densities and histograms are currently drawn horizontally. This is the primary orientation people are used to seeing these in, so they're probably easier to interpret this way. Related to Plot histograms like violin plots #111, I think plotting densities and violins should be self-consistent.
• often one will want to plot many categories. Horizontal orientation is more friendly with this, because one can scroll down to see additional categories, and category names can be stacked more compactly.

The counterpoint is that for plots that use the SampleBased conversion trait, where x indicates the categories and y indicates the samples, it's nice that x maps to the x-axis and y maps to the y-axis.

[piever]

Maybe we could just make it themable? That is, we add some attribute to the theme so that you can do set_theme!(samplebased_orientation = :horizontal) and all your subsequent boxplot, violin plots, etc. are horizontal by default?