It is fine to use templates in tooltips/labels for a non-databound chart. The advantage of using templates in a data-bound chart, however, is that you can display custom text(i.e., obtained from other data source fileds). See such an example is illustrated in the HtmlChart - Client Templates for Series Labels and Tooltips
Regarding defining templates in multiple series - each series must be configured with the corresponding template. You must also consider the following - when creating templates from code behind and there are quotes, they must be escaped with double back slashes and not a single one like it is done in the markup. See details in the Handling Special Symbols
Let me know if there is anything else on this regard.
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