If I understood what you mean, this is the place where value of coins are shown. When you add grade and buying value for a coin, the website will calculate a mean with other users and show in this chart. I don't know exactly how it works (probably there is a help page somewhere in the site explaining that), but I guess it need a certain number of members with that piece for showing a value. If it doesn't show a value, it appears as “-” instead.
It just means that Numista can't calculate a value of the coin based on how much Numista users payed in their own specimen, but you can add the grade of yours withpout any problem
