How can i do or change the vertical bar code visualization to draw negative bars alwais in red colour? Thank you
You will have to write your own plugin to do this right now.
Here are some known plugins in Kibana:https://www.elastic.co/guide/en/kibana/current/known-plugins.html
Here is how you will add a plugin: https://www.elastic.co/guide/en/kibana/current/plugin-development.html