Show line markers if low sample count

[ewels]

It's nice to show markers on line graphs so that it's clear where the exact data points are. However, we don't do this because it doesn't scale well to large sample numbers.

This is a feature proposal to:

• Show line markers if the number of samples is low (eg. default 5, configurable)
• Show line markers if the number of x values for a given sample is 1 (as it's invisible otherwise)