Fix combined chart axis labelling.
Fix combined chart axis labelling.

Closes #311

jkyeung/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jkyeung/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter,jkyeung/XlsxWriter,jmcnamara/XlsxWriter