Drawing diagrams with draw.io
draw.io is a web application that lets you draw flow charts and other diagrams. It works much nicer than office suite tools like LibreOffice Draw.
Its UI is easy to use: Simply drag and drop shapes or arrows from the left-hand side onto your sheet. Drag start/end of arrows to shapes to automatically connect them. You can add text to shapes by selecting them and typing. To add colors, select a shape and use the right-hand panel.
While you can, you don't need to store your charts online. "Offline" storage can be files on your disk (great for adding to a project's repo) or your browser's localStorage. You'll get an XML file that you can load into draw.io again later.
Exporting as image/document supports all kinds of formats, like PNG, SVG, PDF, VSDX, etc.