Line plot fallback for violin plot

If a given metric is run with a repeat of only one and the results are plotted with `tehuti-vis` the plotting will fail because violin plots are not defined for a single value.

This PR adds a fallback to the violin plot function that means it will plot a line instead of a violin on a single value input.

Following this approach also assures compatibility with subsequent benchmarks - if these have repeats >1 then the line will be plotted on the same axes as the violin, allowing for a comparison between them.