Shap summary Plot for binary classification and multiclass

Question

For binary classification, I am getting only a unicolor feature importance plot (i.e., the two classes do not appear individually).

However, for multiclass, I am getting feature importance in different colors. Can someone please explain why I am not getting the same for binary classification?

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thank you for the answer. But I am doing it for two classes (i.e., binary classification). I have also seen people plotting for two classes (please see the figure below). I want to do the same in my case. In binary classification, the "shap_values = explainer.shap_values(X)" only produces one array in binary classification. However, it produces three arrays in multi-class, as I have three.

Answer 1

The importance is drawn in one color, because we have 1 class, why should we draw one value in several colors?

For a multiclass task, shap is considered for each class, so the colors are different. However, you can turn a binary classification into a multiclass classification of 2 classes by representing the target vector as 0= [1,0], 1 = [0,1], where [p1,p2] is belonging to classes 0 and 1, and get 2 colors on the shape graph. But it doesn't make much sense, because we just complicate the task and get no benefit (the probability of class 2 is easily expressed from 1 and vice versa)

Answer 2

In binary classification, the shap values for the two classes, given a feature and observation, are just opposites of each other, so you get no added information by providing both. You can see this, in the aggregate, in your last plot: the red and blue bars are always the same length. I've seen those two-color plots before, but I suspect they're from an older version of the package (or maybe with more custom code?); I've not seen them generated with current shap.

In multiclass classification, more interesting things happen. In your first plot, feature 38 appears to be great at distinguishing classes 0 and 1, but doesn't move the needle on class 2 much. But feature 42 helps distinguish class 2 a fair bit.