I have a histogram showing the distribution of reaction times from $100$ trials worth of data. The range in times is measured in ms and ranges from $70$ ms to $420$ ms. The frequency is displayed on the left y-axis with the max peaking at $28$ occurrences in the $175-210$ ms bin range. The bin sizes, as you could guess, are in $35$ ms sized boxes. I have to add a probability y-axis and a probability density y-axis to the same graph, but I'm not sure how to calculate the probability to see how high in value the axis should go. My lab describes calculating this amount by dividing the scale of the "first axis" by the total number of measurements of my histogram.
I thought it would simply by $35$ for the scale of the "first axis", which I'm assuming is the x-axis, divided by $100$, the number of trials I conducted, but when I start to calculate the probability density, I have to divide the scale for probability by the interval width.
So basically I have to solve the first to solve the second. The problem is I don't know what is the difference between the scale of the first axis and the interval width.
With the assumption of $35/100 = 0.35$ becomes my max for the probability axis, but this doesn't exactly make sense because then the next equation would just be $0.35/35$, which means I'm calling the scale of the first axis the same thing as the interval width.
Could anyone provide some clarification on how I should identify what is the first axis, and how do I find its scale? What's the difference from the interval width?