Counting your quantity in ounces makes sense, I wouldn't change that (I assume each point is a total sales/price combination at a given time?).
If you want to look at the impact of a percentage change in your price, you should take the logarithm of price. A .3 change in log(price) is roughly equivalent to a 30% change in price (more precisely, it is a exp(.3)-1
= .350 = 35% change; the .X = X% approximation breaks down fairly quickly).
That said, you do in fact see a downward slope on your demand curve, which indicates that a drop in price is correlated with an increase in sales, although it's a general negative relationship and there's nothing special about 30% in particular (I wouldn't expect there to be!). So you're pretty much showing what you want (discounting issues about statistical significance, or causal identification - if these prices aren't set randomly this isn't technically showing you a demand curve).