Spectrum range and customised color

[wangjingyu7]

Two ideas of visual effects:

Is it possible to customise the displayed spectrum range? It will make the plot easier and conciser to check in my opinion. A zoom function will be even better?

Another thing related, if only work in a small spectrum range, says red to deep red, is it possible to customise the displayed color, instead of having all plots in red?

[iandobbie]

The first suggestion is already covered in #87