I would suggest a new derived line series to indicate that parts of a curve have been extrapolated/intrapolated. This would be similar to the ThreeColorLine series but the visualization would depend on the horizontal coordinate and not the vertical one.

I have added an implementation and examples to my fork of the project.

The examples look like this:

Compared to a normal LineSeries, the class ExtrapolationLineSeries has three additional properties:

• Intervals: Gets or sets an IEnumerable<Tuple<double, double>> in which the curve is displayed in extra/interpolated style. Can be set to null. If not null, the intervals must be ascending and may not overlap. Otherwise an InvalidOperationException is thrown. Positive and negative infinity are also legal delimiters.

• ExtrapolationColor: Gets or sets the color of the intervals in which the curve is visualized as extra/interpolated. In addition the line style is assigned only to these intervals while the rest of the line is in solid style.

• IgnoreExtraplotationForScaling: If set to true, the extrapolated intervals are not taken into account when the minima and maxima of the dataset are calculated. Hence, the extrapolated line will also not scale the axes.

Is it possible to contribute my work?