When you create a pie chart in Excel 2013, the topmost category in your data shows up in the 12 o'clock position at the top of the chart. Each subsequent category follows in clockwise order. To rotate the chart, starting at a point other than 12 o'clock, change the Angle of First Slice setting.
Select the pie chart, open the Format tab and click Format Selection to open the formatting sidebar.
Set the Angle of First Slice to rotate the pie chart. The number indicates how many degrees clockwise from 12 o'clock to start the first slice. Enter a specific value, drag the slider or click and hold on the up and down arrows to spin the chart to the desired orientation.