I was reading a chapter on differentials in my calculus book, when I came across the graph shown in the image attached to this post. Two questions came to my mind upon seeing this graph: 1) Isn't it technically wrong to label the x-coordinates as x and (x + Δx)? I mean, wouldn't it be more appropriate to label them as a and (a + Δx)? 2) I have always been under the impression that differentials are infinitesimally small. How then can a geometric definition in which differentials are treated as normal real numbers arise?