This is the heatmap section of the gallery. If you want to know more about this kind of chart, visit data-to-viz.com. If you're looking for a simple way to implement it in d3.js, pick an example below.
Add tooltip to heatmap
Add a tooltip that displays the exact value of a cell and whatever other text.
The following heatmap is my starting template for my personal work. It offers most of the feature you need for a good heatmap: colorBrewer colorscale, hover effect, tooltip, title and custom axis. The input dataset has 3 columns: row, column and value. Get the code here.