New view mode: scatter plot among 2 select variables

[breznak]

From a user request,
"I found it particularly helpful to be able to visually relate the data points of
the individual time series, this lets you spot anomalies between the individual
univariate time series (unless they occur with a delay).

Assuming the time series are equally sampled (or they would have to be resampled in that case):

1. Select 2 time series and relate the data points in a scatter plot.
2. For > 2 time series I used a scatter plot matrix.

In addition it is then very helpful to be able to highlight e.g. suspicious data points, and propagate this highlighting between the time series plot and the scatter plots.
"

Ref.: #82