As ballardw says, you might need to insert some 'zero' values into your data.
Here's a small example to demonstrate that. The first one uses a 'missing' value (which doesn't show up in the bar chart), and the second example uses a value of zero (which does show up in the graph)...
data foo; length name $6; name='Fred'; value=.; output; name='Robert'; value=4; output; name='Tim'; value=2; output; run;
proc gchart data=foo; hbar name / type=sum sumvar=value; run;
data foo; length name $6; name='Fred'; value=0; output; name='Robert'; value=4; output; name='Tim'; value=2; output; run;
proc gchart data=foo; hbar name / type=sum sumvar=value; run;
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