<asp:chart ...>
    <series>
        <asp:series name="SampleSeries" xvaluemember="Date" yvaluemember="Value"
            charttype="Line" tooltip="Date: #VALX{d}" />
    </series>
    ...
</asp:chart>

Looking at the tooltip, this should format the X Value to a short date, however when I ran the chart, my tooltip appeared as a number (as it’s represented internally by the charting component).  From my example above, you can see the formatting syntax is fairly simple.  It is just the keyword followed curly braces housing the format string.  You can use any of the standard .NET format strings.  For example, I could format the date like #VALX{MM/dd/yyyy} or if I have a currency for the Y Axis, I could use #VALY{C}.

As you may or may not know, the MS Chart control is based on the Dundas Chart source code 5.5 (more information on the differences here).  With this knowledge, I did a bit of searching and came across this link on the Dundas support site, I finally came up with the solution to my date formatting problem.  The solution is to set the xvaluetype property of the series.  This wasn’t really obvious since I figured the formatting / type from the axis would carry over to the series, but it didn’t.  Anyway, setting the xvaluetype to DateTime, caused the tooltip to format correctly.  Unfortunately this wasn’t shown in the example code in the samples, but hopefully this post will save someone else some time if they encounter this issue.  Here is the final series code, and it now works as expected:

<asp:chart ...>
    <series>
        <asp:series name="SampleSeries" xvaluemember="Date" yvaluemember="Value"
            charttype="Line" tooltip="Date: #VALX{d}" xvaluetype="DateTime" />
    </series>
    ...
</asp:chart>