Plot.decentPositions returning too many positions

[JoelQ]

The calculation of positions doesn't always return the right amount. For a graph with a height set to 100 (with Y values ranging from 1 to 100), this results in the ticks/labels being all bunched up too close to each other.

This seems to be related to the Plot.decentPositions function. Testing it out in the REPL, I get:

> import Plot
> summary = Plot.AxisSummary 1 98 1 98 20 20 100 []
{ min = 1, max = 98, dataMin = 1, dataMax = 98, marginLower = 20, marginUpper = 20, length = 100, all = [] }
    : Plot.AxisSummary
> Plot.decentPositions summary
[10,20,30,40,50,60,70,80,90,100,110] : List Float

Based on the implementation of decentPositions, I'd expect to get 5 positions back, not 10. Digging further, I found that decentPositions calls out to niceInterval with the number of desired positions as an argument.

In my case, decentPositions correctly asks niceInterval to calculate an interval for 5 positions but niceInterval incorrectly returns an interval that results in 10 positions. By removing the magMsdFinal coercion, I was able to make it work with my particular data set but there are a bunch of edge cases it still can't handle.

I think the algorithm of niceInterval needs to be tweeked such that range / interval equals the desired total passed in +/- 1. It might be worth adding a fuzz test around this?

I'm opening this issue in case someone else has run into the same issue and to get help from the community improving the niceInterval algorithm. If I find a good implementation, I will open a PR.

Let me know if there's anything else I can do 😃