Is it possible to be admitted to graduate programs in subjects one minored in in undergraduate?
In the U.S., for graduate work in mathematics, even a minimal major in mathematics is not really adequate preparation for a good graduate program in mathematics. Not enough coursework, which means insufficient exposure to higher-level ideas (assuming, as is apparently mostly the case, there's not much interaction with mathematics outside coursework). Even though various engineering programs require quite a bit of mathematics, in my observation the topics exactly do not get to fourth-year mathematics, so that exactly rigorous analysis, abstract algebra, point-set topology are not seen. The latter are a very minimal preparation for graduate work in mathematics of almost any sort. Still, if there's room for those courses in a "minor", then some minimum thresh-hold would be reached for appearing to be prepared for math grad school.