Histogram bin width does not match actual bin range

[SamuelLKane]

Hi all! I'm trying to use ChartFX's Histogram to plot pre-binned data and noticed a discrepancy that is echoed in the sample code (HistogramSample.java).

What are the issues:

1. The edges of the bin do not reflect the actual range of the bin
2. The data point is not always the center of the bin, instead it can be another point within the bin

Examples

This image shows the green dataset3 from HistogramSample.java which has fixed bins going from 3-4,4-5, and 5-10 (among others). In this case all the visible data points are correctly centered on the bins but the edges of the bins are incorrectly placed (this has the effect of making it look like the data points are incorrectly placed). This is most obvious with the left edge of the bin going from 5-10 which begins at 6, not 5.

This image is from our own implementation using pre-binned data and shows the data points which are placed above the lower bound of a bin, resulting in a histogram that is off by 1/2 a bar. The visible bins are -0.5-10.3, 10.3-21.1, 21.1-31.9, 31.9-42.6.

[RalphSteinhagen]

@SamuelLKane do you use by chance non-equidistant bins (see also discussions in issue #80)?

[SamuelLKane]

@RalphSteinhagen Thanks for the reference. I've poured over issue 80 and confirmed we're using equidistant bins (all are 10.8 in size). We are using the new Histogram(title, numBins, min, max, horizontal) constructor that should create equidistant bins regardless and I confirmed it was via .isEquiDistant(). I swapped to the new Histogram(title, double[]) and was able to get a correctly binned and rendered histogram. I don't know if it's relevant but we also loose the first bin's data in the render when using the numBins, min, max constructor (it only displays bins 1 - numBins as if they were one bin less; bin 1's height in bin 0's place, bin 2's height in bin 1's place, etc.)

[wirew0rm]

This is probably a classic of by 1 error, the convention for the Histogram class is that bin 0 and bin nBins+1 contain the under-/overflow data, so the actual data bins start at index 1.

This was only documented in Histogram#getBinContent(int)I've added a note to the interface itself.
I hope this solves your issue, I've tried with a very simple Histogram and all bins where rendered correctly.

I also addressed the issue of the left shift, which was also present in our implementation, it is now possible to choose whether the given range should be interpreted as center to center or outer bound to outer bound of the first and last bin.

[RalphSteinhagen]

@SamuelLKane we updated the Histogram description and @wirew0rm kindly added the HistogramBasicSample to clarify and illustrate the functionality:

https://github.com/GSI-CS-CO/chart-fx/blob/1f94103f57e1a7e4cdfb97816ea34bac9e8cc206/chartfx-dataset/src/main/java/de/gsi/dataset/Histogram.java#L3-L14

Does this answer your question/issue?

Thanks again for posting this, this helped in clarifying/improving our interface documentation and samples! 👍

[SamuelLKane]

Thanks for being so helpful! The clarity you provided is super helpful as is the sample. We found a work around but as soon as we do another pull and build of your repo I'll let y'all know if I find any more related bugs.

[RalphSteinhagen]

Great! This API convention is coming originally from ROOT, which in turn got it from PAW which in turn got it from a hermit in the swiss alps who claims that this has been passed over eons through word-of-mouth by his ancestors which our little tribe takes for granted now, while other more modern people moved to more modern concepts. ;-)

Let us know if you find things or want to contribute in that area. We are aware that dealing with histograms is still a bit sparse ...

I will close this issue if you are OK with it. Feel free to re-open in case your issue hasn't been solved by this.