# Why aren't bivariate histogram matrices as common as scatter plot matrices

Bivariate histogram matrices give me intuition which categorical variables are related to one another. For example:

I can easily see which variables are related to one another. Indeed, this graphical device isn't brought up as much as scatter plot matrices. Why not? One point of scatter plot matrices is to infer linear correlations between variables in our dataset.

Scatter plot matrices are only useful for continuous variables. So - bivariate histogram matrices seem to be the best bet for categorical variables.

Are there any disadvantages to this approach? Or do you folks use a different technique for visualizing associations between categorical variables?

• I can't see that you are plotting categorical variables here at all. You have three measured variables each divided into 10 bin histograms on the diagonal and heat-maps in off-diagonal cells. Either way, one answer is that this example doesn't encourage me to look at any more, as I don't know how to interpret what I see and most patterns appear random blurs. Can it be that solid black at bottom left and top right encodes low frequencies? It is hard to comment on the merits of a design from one very unconvincing example. Mar 23, 2016 at 16:14
• Scatter plot matrices can be very useful for many non-continuous variables, including binary and counted variables, even ordinally coded variables. There is no assumption of continuity. Mar 23, 2016 at 16:17
• What about for nominal variables? Say that I had 10 nominal varaibles and wanted to find which were correlated with each other? This would work well.
– user46925
Mar 23, 2016 at 16:21
• With nominal variables you could use a scatter plot to see which cross-combinations occur, but there are much better plots. I don't know what kind of "correlation" you have in mind for nominal variables; perhaps you mean some more general kind of association. Mar 23, 2016 at 16:23
• Scatter plots are typically used with ordered variables - I don't think it would work well with nomial variables
– user46925
Mar 23, 2016 at 16:29