We have the function y=x+b.
This is a line with b as y-intercept.
When b gets larger, that graph moves parallel and up in the vertical axis.
That is because b is like a "floor" level when x=0, and, as the slope of the line stays the same, the lines stays parallel.
As b becomes negative, the graph goes in the opposite direction (down in the vertical axis).
The lines stays parallel, because the slope does not change.
