Close together points break 'monotone' cublicLineInterpolation

[nolanthorn]

When using the 'time' xAxes and a normal yAxes, with a y = f(x) type function, I wanted to use 'monotone' as my cubicLineInterpolation because some of my datasets were being drawn not as functions but as relations despite being functions themselves.

However, when there are two points "close enough" (haven't determined what exactly this threshold is) together on the x-axis (as dates), the 'monotone' cubicLineInterpolation option causes the line to stop being drawn and the 'fill' option just kinda wanders down to 0.

See: https://jsfiddle.net/w5kq5mzk/2/

Note the two points in the dataset labelled with comments (both have y = 93), removing either of them or both of them fixes the issue and the graph looks quite nice.

Thanks for the library, hopefully this is an easy fix.