Are you flexible on the 1/2 hour interval to an hour interval?
Crystal has a built in function to group these at the hour level.
Go back to leaving the field as a DATETIME and do not split it into 2 text fields.
It is really straight forward although I do not know what you are graphing...a count of row or a sum of a field in rows or something else. I will assume it is a count of the rows in this sample:
Add a Chart (usually to your Report HEader).
The Chart Expert will open.
Click on the TYPE tab and select LINE chart.
Pick the one you want to use (either Line Chart or Line CHart with markers ...the two on the far left).
Click on DATA tab.
On Change of a add your DATETIME field.
now click on the DATETIEM field that is in the On Change of window. This willa ctivate the "Order" button.
Click on the Oder Button.
Change the This section will be printed: section to "for each hour" and click on OK.
Add your unique record field (usually a primary key) to the "Show value(s)" window.
Click on it to activate it and then click on the "Set Summary Operation" button.
Change the calcualte this summary to "DistinctCOunt".
Close it and chaeck out your graph
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