D3.js Graph Gallery - A collection of charts made with D3.js by Yan Holtz. The gallery includes hundreds of charts with reproducible and editable source code.
Financial Times Visual Vocabulary - A design website from the Financial Times Visual Journalism Team which intendeds to assist designers and journalists choose suitable chart types based on the types of relationship driving their story. The material is also available as a Visual Vocabulary poster via GitHub.
Mapbox: The Guide to Map Design - A guide for designing web maps. Guidance iconsiders both deign elements and design principles: color, typography, iconography, texture, contrast, hierarchy, density, legibility. The guide is available as a downloadable PDF, but the web page is also worth a look as a pointed example of what not to do.
Office for National Statistics (ONS): Presenting Data - Guidance for creating charts and tables and best practice for using colour in visualisation work. Guidance includes: chart design; chart type; chart titles and text; interactive charts, animations and dashboards; tables; using colours; diagrams; datasets.
OrdnanceSurvey GeoDataViz-Toolkit - This toolkit by the Ordnance Survey of Great Britain is intended to help communicate data effectively through the design of compelling visuals. Guidance includes: basemap styles; cartographic design principles; colours; symbols. The repository also contains workshops with step-by-step workbooks and data.
Python Graph Gallery - A curated collection of charts made with Python by Yan Holtz. Charts are organized in 40 sections and include reproducible code. They are mostly made with Matplotlib and Seaborn libraries but others like Plotly are sometimes used.
R Graph Gallery - A gallery of charts made with R by Yan Holtz. The charts are organised into several sections and are provided with source code, focused on the tidyverse and ggplot2 ecosystem.
The Data Visualisation Catalogue - A catalogue of different information visualisation types by Severino Ribecca. The website is intended as a learning and inspiration resource for people working with data visualisation. The collection can be searched by chart function or alphabetically by name.
The Data Viz Project - A filterable gallery of charts types and data visualisations curated by design agency Ferdio. Charts are categorised by family, input, function and chape. Each chart type has a decsription and examples.
Visual Complexity - The website is gallery of network visualisations by Manuel Lima. Though no longer maintained it provides a resource for exploring possibilities for the visualization of complex networks.