Hello there YuriyBo,
Thanks for posting here on the Apple Support Communities! This is a place where other Apple users can submit questions or respond to others to help lend a hand. Let's take a closer look as we do our best to provide some additional insight.
We understand your Apple Pencil isn't producing a straight line when drawing at a diagonal angle. To get started, do you have a case or screen protector on your iPad? If so, be sure to test with them completely removed. Screen protectors can affect the tracking in some cases.
Otherwise, also make sure your screen is completely clean: How to clean your Apple products and and try restarting your iPad if you haven't already: Restart your iPad
Also, be sure your iPad is completely updated:
How to back up your iPhone, iPad, and iPod touch
Update your iPhone, iPad, or iPod touch
We hope this helps get you pointed in the right direction.
Take care.