Select the chart.
Click the Layout tab under Chart Tools.
Click the Axis Titles button
Point to Primary Horizontal Axis Title,
and then click None to hide the axis title,
or click Title Below Axis to display the title below the axis.
Point to Primary Vertical Axis Title,
and then click None to hide the axis title,
click Rotated Title to display the axis title rotated,
click Vertical Title to display the axis title vertical or
click Horizontal Title to display the axis title horizontal.