how can I subtract 1375 - 1875 and show me the difference in the graph ?
You may want to refer to this similar discuss thread for guidance. Looking at the two github issues related to this request, I'm afraid this feature hasn't been implemented yet! Having said that, there are a few good suggestions in the linked thread which may be of use!
May be this plugin could help you : https://github.com/datasweet/kibana-datasweet-formula
It's not working on kibana 6.5 yet.