implemented null value support

[nniclas]

Explanation About What Code Achieves:

Null data support added to line chart
New code renders line paths with gaps using "null" values.

Screenshots/GIFs:

Steps To Test:

Add a line chart and set some null values to datasets

TODOs:

• A simple interpolation is added but not tested fully, can be enabled in AxisChart.js.
• Null data drawing logics are dependent of the dataset real values, a drawback is they need to be provided through the call chain, some advice is needed if the desire is to make use of the "yPositions" property.
• Note: some refactoring has been done in draw-utils spline function

[coveralls]

Pull Request Test Coverage Report for Build 134

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 38.158%

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Totals
Change from base Build 133:	0.0%
Covered Lines:	29
Relevant Lines:	62

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💛 - Coveralls

[y-nk]

Any chance to see that PR merged in a near future ?

[scmmishra]

Is there any generic use-case for such a chart?

[nniclas]

@scmmishra As many other chart libs has this feature I believe it could be a nice addition, however you may just go ahead and close this if it makes sense. 👍

[jakebott]

@scmmishra I would also be interested in seeing this pull request merged. We use Frappe charts to plot sensor data, but as the communication to the sensor is not 100% reliable and as a result we occasionally have missed data points - null value support would allow us to show the missing data.

Also as a side note: this would fix #98

[scmmishra]

I'll review this and try to get this release soon

[jz222]

I would also be happy if this was merged. It would be great to have a general option for line charts to render smooth instead of spiky lines.

[scmmishra]

@nniclas can you complete test interpolate feature? From my testing it seems like its good to merge. Let me know we can enable the option.

[arthyn]

+1 for getting this merged, really like what I see from the library, but it would be nice to be able to pad out the labels

[danekolsen]

Would love to see this get merged and put out in a new version!