It's easy to recognize visually when an orthogonal coordinate system has its axes in the same scale. See, for instance, the following image. But I'm trying to write down a precise definition of it.
After searching the usual channels (Google Scholar, Google Books), my impression is that it seems this knowledge is implicit in High School teaching, that is, the explanation given is mainly visual: the teacher shows an example of an orthogonal coordinate system with the same scale on both axes and another example with different scales. Done. Some people define "the axes are on the same scale if they have the same unit" but, then, what does it mean "to have the same unit"? I'm looking for a precise definition accessible to Secondary School students.
So, my three questions are:
(1) Do you know a precise definition for "axes on the same scale" accessible to Secondary School students?
(2) Do you know some school textbook or a scientific article/book where such definition is presented or discussed?
(3) What do you think about this definition: "We say that an orthogonal coordinate system of the plane has the x and y axes in the same scale if the segment joining (0, 0) to (1, 0) has the same length as the segment joining (0, 0) and (0, 1) when measured with the same ruler.".
While in Analytic Geometry it is always assumed the axes are on the same scale, when studying functions or Statistics, axes on different scales is a necessity.