title: Count all categories in pandas data frame and add their values
body: <p>I've got a data frame that looks something along these lines:</p>
  
  <pre><code>           Dog_breed       Dog_name             Points 
        ============================================================
  Monday        Pug            George               12
  Tuesday       Poodle         Fido                -15
  Wednesday     Pug            Buddy                 8
  Thursday      Dachshund      Lyla                -10
  Friday        Poodle         Remi                  9
  Saturday      Pug            Kermit               -5
  Sunday        Beagle         Sara                  3
  </code></pre>
  
  <p>For every breed of dog (i.e., category), I need to get the sum of the points — for some rows, it's positive, and for some, it's negative — and the number of dogs of that breed. </p>
  
  <p>The goal is to add up all the points for each breed, count the number of times each breed occurs in the original data set, and end up with a data frame that looks like this:</p>
  
  <pre><code>             Quantity     Overall_point_change
            ============================================
  Pug             3                  15
  Poodle          2                  -6
  Dachshund       1                 -10
  Beagle          1                   3
  </code></pre>
  
  <p>Can pandas do this, or does it require a loop? Really struggling to wrap my head around this — can anyone recommend a way to set this up?</p>