Replace Float with Double as backing datatype for chart data

[trein]

This PR prevents rounding errors when charts are constructed with data composed of Double values.

[gpbl]

Thanks! Could you provide an example to replicate the issue this PR is fixing?

[trein]

Hi @gpbl,

Absolutely. I was plotting a set of Double points representing timestamps (Date#timeIntervalSince1970) and currency values. My timestamps values where dividing a single hour into 60 intervals (i.e. one point per minute). After playing with SwiftChart, I noticed that my timestamps points started overlapping.

After some investigation, I realized that SwiftChart exposes an initializer ChartSeries(data: [(x: Double, y: Double)]) but converts data internally into [(x: Float, y: Float)]. Float is unable to express a Double accurately due differences in internal representation (Float uses 32 bits and Double uses 64 bits).

Bottom line, my timestamps values were losing precision and ended up creating overlapping points in the graph. Here is a sample of my timestamps values showing the problem:

Original timestamp values: [1514895300.0, 1514895360.0, 1514895420.0, 1514895480.0, 1514895540.0, 1514895600.0, 1514895660.0, 1514895720.0, 1514895780.0, 1514895840.0, 1514895900.0, 1514895960.0, 1514896020.0, 1514896080.0, 1514896140.0, 1514896200.0, 1514896260.0, 1514896320.0, 1514896380.0, 1514896440.0, 1514896500.0, 1514896560.0, 1514896620.0, 1514896680.0, 1514896740.0, 1514896800.0, 1514896860.0, 1514896920.0, 1514896980.0, 1514897040.0, 1514897100.0, 1514897160.0, 1514897220.0, 1514897280.0, 1514897340.0, 1514897400.0, 1514897460.0, 1514897520.0, 1514897580.0, 1514897640.0, 1514897700.0, 1514897760.0, 1514897820.0, 1514897880.0, 1514897940.0, 1514898000.0, 1514898060.0, 1514898120.0, 1514898180.0, 1514898240.0, 1514898300.0, 1514898360.0, 1514898420.0, 1514898480.0, 1514898540.0, 1514898600.0, 1514898660.0, 1514898720.0, 1514898780.0, 1514898840.0, 1514898900.0]

Converted timestamp values: [1.51489536e+09, 1.51489536e+09, 1.51489536e+09, 1.51489549e+09, 1.51489549e+09, 1.51489562e+09, 1.51489562e+09, 1.51489574e+09, 1.51489574e+09, 1.51489587e+09, 1.51489587e+09, 1.514896e+09, 1.514896e+09, 1.51489613e+09, 1.51489613e+09, 1.51489626e+09, 1.51489626e+09, 1.51489638e+09, 1.51489638e+09, 1.51489638e+09, 1.51489651e+09, 1.51489651e+09, 1.51489664e+09, 1.51489664e+09, 1.51489677e+09, 1.51489677e+09, 1.5148969e+09, 1.5148969e+09, 1.51489702e+09, 1.51489702e+09, 1.51489715e+09, 1.51489715e+09, 1.51489728e+09, 1.51489728e+09, 1.51489728e+09, 1.51489741e+09, 1.51489741e+09, 1.51489754e+09, 1.51489754e+09, 1.51489766e+09, 1.51489766e+09, 1.51489779e+09, 1.51489779e+09, 1.51489792e+09, 1.51489792e+09, 1.51489805e+09, 1.51489805e+09, 1.51489818e+09, 1.51489818e+09, 1.51489818e+09, 1.5148983e+09, 1.5148983e+09, 1.51489843e+09, 1.51489843e+09, 1.51489856e+09, 1.51489856e+09, 1.51489869e+09, 1.51489869e+09, 1.51489882e+09, 1.51489882e+09, 1.51489894e+09]

Take the first two values of each stream. Although they are represented by two distinct Double numbers
[1514895300.0, 1514895360.0] they ended up mapped to same values [1.51489536e+09, 1.51489536e+09] after conversion to Float.

Please, let me know if it helps.

[trein]

Hi @gpbl . Just a correction here. This is not an enhancement. This is a bug fix. Anyone using this constructor will potentially experience representation issues. Simply put, Double variables cannot be represented as Float.

[gpbl]

Thanks for your help! I think this is the time to cut a v1.0 release 👍🏽