Thanks for sending that screenshot- definitely clearer what is happening.
I hate to be the bearer of bad news, but the behavior of not all values being shown in the image is expected for reports with a lot of breakdowns. I see how this is not ideal, especially if you’re relying on Slack subscriptions to share reporting with teammates who don’t have direct access to Mixpanel. I've gone ahead and logged a feature request with the product team for your specific use case.
There isn’t a better setting or workaround for increasing the Slack subscription image size today. If you aren't tied to a bar chart, the metric view image contains up to 8 segments, which may work for this specific example. Outside of that, you could split/simplify the chart so the subscribed snapshot is fully captured in the image. Another option would be to use a password-protected Public Board, which would allow the user to see the full report.