You can change many visual aspects of graphs using this dialog, which is accessible from the any graphs page under Actions > Edit Legacy Graph Styles.
Edit graph styles from the Actions menu
Graph style options
Show markers in graphs
When Show Markers is selected, each data point wil be represented with a circular marker.
Show Y-axis grid in graphs
If selected, the vertical axis scale markings will be shown.
Show data point values in graphs
When this option is selected, each data point will be labeled with its underlying numerical value.
Show Data Point Values selected
Select line width for graphs
Choose the thickness of graph lines in MyKeynote line graphs (1 thinnest - 5 thickest).
This setting also affects the size of markers shown in graphs, such as scatter plots.
Select background color for graphs
Choose the color you want to appear behind MyKeynote graphs. Choices are None (white), Light Grey, and Light Blue.
Select background border for graphs
Select the style for graph backround border.
Use bar graph instead of line graph
Choose this option to create bar graphs instead of line graphs.
Use area graph for time history when single measurement/page is selected
Choose between area graph style and normal.
Use 3D view for graphs
When this is selected, line graphs will have a 3D effect.