title: R sum on rows based on day
body: <p>I have a dataframe containing sells by day, but as there are many values for one day, so i'd like to rebuilt my dataframe to have just one value per day which is the sum of all the sells of this day.</p>
  
  <p>what i have: </p>
  
  <p><a href="https://i.stack.imgur.com/hYtrr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hYtrr.png" alt="what i have"></a></p>
  
  <p>what i'd like to have:</p>
  
  <p><a href="https://i.stack.imgur.com/TF7Aw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TF7Aw.png" alt="enter what i&#39;d like to have here"></a></p>
  
  <p>(the values are not well calculated but it's for the example) </p>
  
  <p>I tried agregate and functions like this but it dosn't work and I dont know how to do this...</p>
  
  <p>Thanks for help</p>