Not exactly sure what you mean by graph 'O26'. Are you referring to a sparkline?
You also havn't mentioned whether the values in B2 and B3 are percentages already or not. This makes a big difference because 15% is 5% higher than 10% but 15 is 40% higher than 10. I am going to assume that the numbers are already percentages as this makes the if statement much simpler but it can easily be adapted to calculate either.
But sending an email through app script is relatively straight forward.
function sendEmail(recipient, b2, b3, y,o26) {
var subject = 'Automated Email;
var body = 'Whatever you want the email to say and include o26 (whatever that is)';
if (b2-b3>y) {
MailApp.sendEmail(recipient, subject, body);
}
}