How to represent quantity in an X / Y axis grid without taking too much space?

We have a graph that displays the symptoms (y axis) and the ages of people who have them (x axis). The bigger the circle, means more people of the same age range share the same symptoms.

Example of data that can be plotted in the graph:

30 Patients, Aged 25-34, have Headache symptoms. (bigger circle)

2 Patients, Aged 55+, have Rash symptoms. (very small circle)

My question is, is there a better way to represent quantity in this graph without resorting to circle size? Because the circles may overlap and make it hard for people to see where each circle is located. And the bigger circles take too much space.