Expected Points (xPTS) are based on xG. A player's xG, xA and xG against (clean sheets) are multiplied based on FPL's scoring rules for each position (i.e. midfielders score 5 points for a goal, 1 point for a clean sheet and 3 points for an assist). For example, a forward with 7.8 xG and 2.1 xA would have a xPTS total of (7.8 x 4) + (2.1 x 3) = 37.5. The total xPTS for each player is then divided by the total minutes played x 90 = per 90 mins. Expected bonus points are also included based on xG values. Penalties are accounted for by crediting penalty takers with the avg. number of penalties per team per season. xPTS does not consider points for minutes played, yellow cards or other scoring ways in FPL.
The 80th percentile is calculated based on the aforementioned methodology for prior FPL seasons. It can help identify 'elite' (top 20%) players.
'Value' picks are those who are above the historical 80th percentile while cost picks are below. This does not mean 'Cost' (situational) picks cannot be good selections in certain GWs - though across a full season better value is elsewhere.

Additional Notes:
The faded grey dots on the plot are players from previous seasons. GKs are excluced as a signigicant proportion of their points are generated from saves.