R - How to make a plot with two variables on the horizontal axis? : <p>I'm a rookie in R.</p>

<p>This is my date frame:</p>

<pre><code> I  UserID | hour | min | velocity
   #1    1        0     0      12
   #2    1        0     30     20
   #3    1        1     0      19
   #4    1        1     30     11
   #5    1        2     0      12
   #6    1        2     30     7
   ..   ...     ...    ...   ....
   #10   2        0     0      142
   #11   2        0     30     201
   #12   2        1     0      129
   #13   2        1     30     111
   ..   ...     ...    ...   ....
</code></pre>

<p>I put the Userid column as a factor.</p>

<p>My question is how can I make a linear graph using as the horizontal axis the hour and minute column and velocity as the vertical axis?</p>
