Centered reference line labels

Reference line labels are currently placed next to the line and collision detection is used to make sure that they do not overlap. When there are many reference lines in a visualization that sometimes makes it very hard to see which label belongs to which line. It should be possible to configure the labels so that they are centered on the line, similarly to the configuration option that exist for markers.

Sample use cases:

• When a Scatterplot is configured to show a cumulative distribution functions horisontal reference lines can be used to show the 1st, 10th, 25th, 50th,75th,and the 90th percentile, but with the current reference line label algorithm the percentiles show up in the wrong order when the plot area is small.

• When a Scatterplot or line chart is used to show something happening over time, vertical reference lines can be used to show specific events so that it's possible to visually correlate the events with the time series. In this case the exact placement is also very important.