title: ggplot bar chart for frequency multiply by sales
body: <p>my data has a list of orders consist of
  order number, product category, number of sales</p>
  <p>each order can contain multiple number of sales</p>
  <p>I need to create a bar chart of sales by product category
  , Instead of just counting the number of orders for each product category.
  I need to count number of orders multiply by their respective number of sales.
  how do I write the code? thanks</p>