Brian Graham netted a double to send Partick Thistle into the cinch Premiership play-off semi-final with a 2-1 victory over Airdrie at Firhill.The Thistle captain scored from close range in each half to take his season tally to 26 and earn his side a 4-3 aggregate victory.Arron Lyall pulled a goal back for the Diamonds and the visitors had plenty of possession and pressure – the corner count was 12-0 in their favour.But the Jags defence was resolute and Callum Fordyce hit the bar with the best chance to level as Kris Doolan’s side booked another two-legged encounter with Raith Rovers next week.Airdrie, who were missing captain Adam Frizzell, gave as good as they got in the opening stages but Thistle opened the scoring in the 18th minute after a long spell of possession.Kerr McInroy put Airdrie on the back foot when he zipped a pass through the lines and Aidan Fitzpatrick got beyond his man to deliver a low ball for Graham to tap home from six yards.The visiting defence immediately appealed for offside and footage suggested they were right but it was tight and the assistant referee gave the striker the benefit of the doubt.Airdrie were struggling to feed their front players but they did exert some pressure late in the half, however a series of set-pieces led nowhere.Visiting manager Rhys McCabe made a half-time switch as Lewis McGregor came on for forward Gabby McGill, who had suffered a shoulder injury in the first leg.The Jags doubled their lead within two minutes of the restart. Scott Robinson benefited from a break of the ball and delivered a low cross which Graham turned home after beating goalkeeper Robbie Hemfrey to the ball.Airdrie had lost 4-0 at Firhill last month, the result proving the difference between the teams in the table, and there appeared little way back given their lack of cutting edge.But they quickly got back in the game in the 55th minute after a penetrating pass down the left side of the Thistle defence. Aaron Muirhead could only head McGregor’s cross into the feet of Lyall and the midfielder slotted the ball into the bottom corner from 12 yards.The game remained finely balanced although Thistle had chances to ease the nerves of the home supporters in the 6,531 crowd. Hemfrey saved from McInroy after the former Airdrie midfielder found a way through a crowded goalmouth and Cammy Ballantyne threw himself in front of Fitzpatrick’s strike.Airdrie began to push the hosts back. McGregor’s strike was deflected wide, Fordyce volleyed off the top of the bar after a corner and Nikolay Todorov had a decent penalty claim denied by the well-placed Colin Steven following Daniel O’Reilly’s risky slide tackle.Fitzpatrick missed a one-on-one to keep the tension going but Airdrie’s chances ended when Mason Hancock headed over seven minutes into stoppage time.