I have a problem that is:

f(0)=1; lim f(x)=3 as x approaches 0 from the left

lim f(x)=1 as x approaches 0 from the right

I know how to graph looks like, when i did it, but in the back of the book

lim f(x)=3 as x approaches 0 from the left<---- it has a hole

lim f(x)=1 as x approaches 0 from the right<----has a solid dot.

Why are the lines of these functions have dark dots, and some have holes??