List of free diagram stuff for developerππ½
If you're looking for all programming tools around diagram (not just free). Check this page List of diagram Programming Tools

Viezly
Viezly helps you review code easier with enhanced navigation and review helpers
Tag: code-review, clean-code, diagram
Free for open source repositories Free for personal usage
Draw IO
Free diagram software for making flowcharts, process diagrams, org charts, UML, ER and network diagrams.
Tag: diagram, flowchart
Open Source Free No login or registration required
Line Graph Maker | Create a line chart for free
Line graph maker online. Create a line graph for free with easy to use tools and download the line graph as jpg or png file. Customize line graph according to your choice.
Tag: Graphs, Charts, Data-visualization, Diagram