How can I change the graph so that it shows more or fewer grid lines?

The spacing of the grid lines in Logger Pro are automatically determined based on the axes limits in the graph. You can adjust the axes limits to change the spacing, but here is not a way to specify that a tick mark be shown every x units.

You can adjust the graph options by double-clicking anywhere in the graph and adjusting the color, style, and visibility for both major and minor ticks.

By default, minor ticks do not have lines, so if you enable lines for minor ticks, you will get more dense lines.

You can also remove all lines if you set line style for both ticks to be "No Line." Removing all lines is often desirable for printing or publication.

In the axes options tab, you can also make all values major ticks.