The logarithmic scale ranges from undefined (close to zero) to MaxY. There is no negative output from a log function. Unfortunately the TMS FNC Chart does not support symmetrical logarithmic scales. We'll add this on our feature request list.

Is it possible to somehow make a custom Y axis with a logarithmic scale, or is this also not possible? Tell me which way to dig, this is very necessary. Thanks for the answer!

It's not possible to use a custom logarithmic scale. When enabling the logarithmic scale feature, the range is limited. So right now, without us making code changes it will not be possible unfortunately.

I wrote a little about something else.
Turn off the built-in logarithmic scale and make a custom axis with a logarithmic scale without using the built-in one.

Just to be sure, do you have large negative values or values between 0 and 1? Note that the Log10() does not accept negative values therefore we cannot render a logarithmic scale below zero. You can have a linear scale below zero ofcourse, regardless of the range of values you feed to the series. If you wish, you can post some values and maybe sketch or demonstrate what you want, so we can help out in the right direction, ofcourse knowing the limitations. There are ways to alter the values being displayed in the chart whilst keeping the original values in the series.