Plot average line

plotAverage adds an average line into plotted data.

By default, plotAverage
- calculates the average along y (for given or estimated points along x)
- plots errorbars with the standard error of the mean
- discards outliers to calculate the robust average
- plots the average line into the data's axes (as opposed to a new figure)

All these options can be changed.

plotAverage returns the handles to the average line and the error bars as well as the x and y data of the line.