RE: Changing X-Axis Labeling 

You can use the data labels of an additional data series to mimic the x-axis labels.
The labels displayed are controlled simple by entering a value in column B of the data sheet.

Select the 'Axis Labels' series in the chart and change the chart type to Line.
Add data labels displaying the Category value.
Format the data labels to be positioned Below and change the alignment.
Format the series line to have no border or markers.

You will also have to format the x axis to remove the tick labels.