I think that the original issue in this thread is now resolved. It seems that the chart did not support dynamic font-size changes at the time of the post, but we implemented this about a year ago.
However, note that what we implemented last year concerns only the FontSize property of the direct chart elements, such as the chart itself, the axis, the series and so on. If you were to change the font size or visibility within a custom template, the change will not be detected and the chart will not update.
I want to elaborate this a little bit. The chart caches many things, amongst which is the size of the axis title. If you change the FontSize property of the chart, the Title or the TitleTemplate, the chart will detect this change and will redraw properly. However if you change something in the template, the chart will not update, because it already cached some size and it is going to use it from now on.
With this being said, it is clear that you should not use a visibility converter within the template, but clear the Title property of the axis if you do not want it displayed.
Let us know if you need more information.
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