Insert special symbols into a Grid Chart

[#1]

I’ve the following grid chart that represents the route of a customer in a office.

I've one problem with this route:

How Can I know what is the beggining point of the path?

What I think to solve is to to introduce some symbols (not the bubbles) to guide the user. Something like this:

Where we can see what is the start point (A), for example. Anyone knows how can I do that?

Thanks!