Compared Graph?

Hi, i’m going to make a graph for Power Consumption Monitoring project. The idea is comparing " Last month Values" and “This month Value”, electricity cost in particular. This values expands everyday and reset at the end of the month. So i want to draw a graph which contains both " Last month line" and “This month line”. The problem is they are two diffirent variables. I uses Pzem004t hardware, and in MQTT message, “Last month value” is a constant, so it will show a straight line on graph.
I have some idea, save everysingle value to a spreadsheet then draw it, stop drawing the line at the end of the month then draw a new line override it, v.v… but i dont know how to implement this.
Please help me and sorry for my bad English.
Thank you!