Hello muszek,
First, the site and tool you've created looks great. If possible in the future, I think it would be a good idea to be able to add a "target BP", both Sustolic and Diastolic, onto the chart, and have it adjustable per user. Maybe add it onto the tool area bove the chart where you now have the date selection.

This way the BP history can be viewed at a glance in relation to the target/goal without having to figure out mentally where that may be.

Thanks,
Larry