I somehow managed to generate .xlsx with charts with C# and the SDK.
But I don't have a clue on how to make a line chart with multiple Y-axes.
Does anyone has any idea?
At first, I was thinking you wanted multiple lines (series) on the chart, but I think you are asking how to get a second value axes on the chart. If that's correct, then the easiest way to do this is to make a copy of your existing chart, then modify it in Excel to add the second axis, save and then compare. In Excel, right-click on the line you want to use on a second axis, select Format Data Series..., then select Secondary Axis. Now you can compare the two versions to see what needs to be changed.
Eric White has an excellent video that details this process in his blog. His example is for a wordprocessing document, but it works just as well for spreadsheet documents.