It's not a bug in Charts extension, it's a bug in the Graphs module on Commons. I've reported it about a month ago.

The problem is that Lua doesn't store a nil value in its array. Therefore, during the transformation from Lua arrays to JSON tables, it moves a value from the next cell after nil to the cell with a nil value (or from y2 to y if we talk about Module:Graph:Chart). It would be useful if the Lua transformer to JSON had a magic word to denote that a cell has a null value. For more information, see https://www.mediawiki.org/wiki/Extension:Chart/Transforms#Null/nil . As for now, I think that the only way to fix, is to transform nil values to 0 or "", but this also significantly distorts the charts with big drops to the bottom if the line chart has thousands on its y-axis.

This bug is related to T388434. Therefore, I'm also adding JsonConfig to this task.

If I understand the issue correctly, adding a 0 can't be the solution.

Let's think on two lines, the first one has value A=6 and C=8. The second one has values A=9, B=10 and C=7. Adding a B=0 to the first one would change the most logical value for that point, something around 7. The graph needs to know that there is no point at B, it should go directly from A to C, while showing A, B and C for the second line. I understand that a null value is the best solution.

In T413353#11483997, @Theklan wrote:

If I understand the issue correctly, adding a 0 can't be the solution.

Let's think on two lines, the first one has value A=6 and C=8. The second one has values A=9, B=10 and C=7. Adding a B=0 to the first one would change the most logical value for that point, something around 7. The graph needs to know that there is no point at B, it should go directly from A to C, while showing A, B and C for the second line. I understand that a null value is the best solution.

I agree with that. However, as for now, charts with null values don't make a continuous line. Here is a chart with a tab (source: https://commons.wikimedia.org/wiki/Data:Competitions:Views_from_Ukraine.tab ; unused tab that I used for testing behavior with null cell) with a null value in the blue line:

Good example. Then null is not a solution. It solves part of the issue (taking fake numbers) but the line should be continous. I assume that each line should be taken as completely independent.

If we're sure the bug is in https://commons.wikimedia.org/wiki/Module:Graph:Chart , and not in https://www.mediawiki.org/wiki/Extension:Chart or https://www.mediawiki.org/wiki/Extension:JsonConfig , then this ticket may be filed in the wrong place and we might want to close it as invalid.

In T413353#11484042, @Novem_Linguae wrote:

If we're sure the bug is in https://commons.wikimedia.org/wiki/Module:Graph:Chart , and not in https://www.mediawiki.org/wiki/Extension:Chart or https://www.mediawiki.org/wiki/Extension:JsonConfig , then this ticket may be filed in the wrong place and we might want to close it as invalid.

I think it can be reformatted as a feature request with a text like: Create a way to explicitly mark a cell as null in a Lua array that will be transformed by JSON config to a null cell in a JSON table. Because, as for now, I haven't found a way to mark a certain cell as null in Lua that would transform to null in JSON table.

I see even two feature requests here -- 1) adding support in Extension:Chart for null or NaN values -- making continuous lines by ignoring them, 2) adding support in Extension:JsonConfig for producing null values from Lua (and obviously 3) patching Module:Graph:Chart in the required way).