Henk De Haan:
Thank you for your post.
At this time, it is not possible to add vertical gridlines to charting. FileMaker Pro currently takes the highest and lowest values and bases the chart gridlines based on those values.
I recommend that you enter this suggestion into our Feature Requests web form at:
The entries into this web form populate a database file that is hosted and monitored by Development and Product Management. All entries are discussed and considered for possible inclusion in a future release. Although I could copy your post and paste it into the web form, there are a couple of questions asked that only you can answer.