Feature request: trendoffset or trendrange

[olobarius]

It would be nice to be able to do the trendline just for some part or range of the data.

In my case just for the last 10 elements. I did a quick hack and implemented a trendoffset parameter.
If > 0 it skips the first n elements, if < 0 it uses the last n elements.

The patch alters the firstIndex initialisation accordingly.
My code looks like this:
...
let fitter = new LineFitter();

// implement trendoffset paramater, alters firstIndex.
let trendoffset = dataset.trendlineLinear.trendoffset || 0;
if(Math.abs(trendoffset) >= dataset.data.length) trendoffset = 0;
let firstIndex = ((trendoffset<0)?dataset.data.length:0) + trendoffset + dataset.data.slice(trendoffset).findIndex((d) => {
    return d !== undefined && d !== null;
});

let lastIndex = dataset.data.length - 1;

...

The config looks like this:
trendlineLinear: {
trendoffset: -10,
...
},

[olobarius]

I missed to post a modified line above. The forEach loop needs to skipp till firstIndex is reached, so it looks like this:

    dataset.data.forEach((data, index) => {
        if (data == null || index < firstIndex) return;