Prevent values like -0 in xdot output

Two graphs in the shapes regression test reference files (invhouse and invtrapezium) contained a -0 coordinate. The Linux build produced the same results, but the Windows build didn't, causing the regression tests to fail. To prevent values like -0 in the xdot output, a comparison is added just before formatting the number.

I used the same comparison as I did before (but with a smaller value). Is this a good method for now?