If you really really need SGPLOT to summarize the data by day, you need to use VBAR. Then the x- axis will be discrete, and the axis will try to show each value, thinned by collisions. To avoid this you will need a format to make the non- labeled values to blank. Setting the VALUES option will subset axis range.
If you can summarize the data by day prior to SGPLOT, using MEANS, you can use a needle plot with a real time axis. Then, you can set the values as mentioned above. You could also use a HIGHLOW plot to get a bar like feel.
With SAS 9.40M3, you have best of both worlds, and you can use VBAR with a linear axis by setting XAXIS TYPE=Linear;
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