A 2D Histogram, or Density Plot, visualizes the joint distribution of two different axes: it lets you see how much of one thing there is, given another.
Consider the scenario in the scene below. There are two columns of raw data. On the left side, we have built individual histograms of each one--and they both look a bit like normal curves. Only the 2D histogram reveals that there is a connection between the two dimensions: that high values in one axis go with low values on another.
A 2D histogram is sort of like a scatterplot, but can show clusters of points. Even if there are a few thousand points, or the points overlap, the density plot can still show them all.
How to use
First, add the Histogram 2D App from the Office Store. (Here are the basic instructions for getting started with Apps for Office.)
Click Sample Data to add two columns of sample data to your spreadsheet. Click the button again to see categorical-vs-numerical data, which can look at the frequency of sales of "apples" compare to "pears". Click a third time to see sample data for categorical-vs-categorical.
Make sure you have data in exactly two columns selected, and click Set Data to set the data in the histogram to the column.
The two sliders control the number of bins: drag it left to make fewer, big bins; drag right to make more, small ones.
The Color box allows you to select a color for the squares in the chart.
The histogram can view either categorical data (such as text labels) or continuous values. The Category checkbox forces the system to choose one bin for each distinct value in the dataset.