Question: Could you explain what is a difference between readable vs legible? 

Which should I use when I want to say "This diagram is easily readable/legible" while meaning - "It is very easy to read and understand meaning of this diagram"?

Answer: 
Dictionaries usually give these as synonyms, but there is a nuance: *legible* tends to refer only to the presentation, e.g. penmanship, while *readable* is broader. If what I'm looking at is a hard-to-read scrawl, that's illegible; if it's nicely typed but the grammar and punctuation are all wrong, such that it's hard for me to understand, that's unreadable.
Both **legible** and **readable** have sense of "clear enough to read".

> 
> Her handwriting was clearly legible.  
> 
>  The figures should be clearly readable
> 
> 
> 

But, **readable** also may mean "easy, interesting and enjoyable to read".