Why are the values at the start of the Chart's time period not zero?

The difference from 0 at the start is how much the stock price changed on that first day. This lets you set a start date of May 1, for example, to see how stocks moved on that first day of May without having to set the start date as April 30. So you can see how each stock moved on the first day, as well as how they performed over the entire period.