How to hide the grid background on some bar

[HsiaoAi]

The X axis data is date array, include 30 days(count = 30)
so I set the label count is 30 and then use customs valuer formatter to show only five labels,
and default the chart has 30 bars with grid background, I'd like to ask how to hide other grid background when the label is not showing(return string is "") as image,
I tried to subclass axisRenderer, but when I init always get memory error.
How can i subclass right and override the drawGridLine(), thank you !

[jjatie]

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