Think on the
ScrollPagerTool.SubChartTChart as a different TChart, with its series and axes. Indeed, there are some shorthands like ScrollPagerTool.Series to access the first series, but it's basically a SubChart.When you assign a series to ScrollPagerTool, the series is cloned into it, but not the custom axes. And you can't assign a custom axis from a TChart to another TChart (or SubChart). If you need a custom axis in the SubChart, you can create it manually.
