Add additional y-axis lines to diagrams

[hwesselmann]

I needed a way to draw percentiles lines into a cycle time scatterplot diagram I created. The code works for my use case and did not break the tests, so I thought it might be valuable for other users as well.

Three options were added: line position, line colour and line label. All options take an array of values and plot those onto the diagram.

[hwesselmann]

The build has failed because rbx could not be built. Maybe there is a configuration problem with the ci job?