/* When we call selectAll and data, we create a "data join", we join data (in this case our 5 apples (array) and some elements (DOM elements in this case: our circle)

We have 3 case > Enter : we only have data, no elements

Update : we have both data and elements Exit : we only have elements but no data

In d3 we need to explicitly call thos 3 functions (.enter, .update, .exit)

selectAll give the elements part, .data give the data (always an array)

thanks to that d3 know exactly how much element are on the enter, update and exit */