The following VBA enabled spreadsheet allows the user to produce the N-M interaction plots and strain diagrams for a given concrete rectangular column cross section. This was used in a recent column design verification post http://communities.bentley.com/products/structural/structural_analysis___design/w/structural_analysis_and_design__wiki/17799.column-design-verification-as3600-09-ram-concrete.
The N/M values are obtained by resolving the forces and moments at each trial neutral axis depth.
Please see video below for demonstration:
The tool can be downloaded from here (note that its a Macro Enabled Spreadsheet ):