fdo77216-Charts-Scattered chart: Chart gets distorted on RT

In case there is some text entered in place of X coordinates for scatter chart
then x coordinates should be taken as 1,2,3....
MS Word does the same thing

Change-Id: I1db0fd64c6ac0f4d5e77a9676812f5e26577ecf6
Reviewed-on: https://gerrit.libreoffice.org/9011
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>