<table border="0" cellborder="1" cellpadding="5">
<tr>
<td colspan="2" align="left">d</td>
</tr>
<tr>
<td align="left" width="50">ct</td>
<td align="left">1</td>
</tr>
<tr>
<td align="left">wt</td>
<td align="left">0</td>
</tr>
</table>
The Graphviz output:

The expected output:

Try adding
cellspacing="0"to yourtabletagAlso take a look at this: http://www.graphviz.org/doc/info/shapes.html#html