To change the axis or grid line style:
Double-click the chart with your left mouse button.
Right-click the chart once. A menu will appear.
Click Chart Designer. The Chart Designer window will open.
In the box on the left, click Category (X) Axis to format the X-axis (or Value (Y) Axis to format the Y-Axis, or Category (Z) axis to format the Z-axis).
Click the Pens tab. Here, you can change the width and color of your axis line. You can also change the style (solid, dashed, etc.), width, and color of your major and minor grid lines.
When you are done, click Apply, and then OK.