How do we plot \$u(t−τ)\$
The step function is one when it's argument is greater than or equal to zero, as the wiki page you linked to says.

Therefore, to determine when the function outputs a 1, you need to determine when it's argument meets the above definition. So you have the equation 

$$t-\tau \ge 0$$

When is this true? When 

$$t \ge \tau$$

So really \$u(t-r)\$ is just a delay with respect to the variable t. There are some very good interactive tools for understanding this kind of stuff here. As you can see in the "Joy of Convolution" applet on the linked page, the step function is frequently used with respect to tau, in which case it is time-reversed as you mentioned.