Put the pineapple chunks, rosemary and tequila in the cocktail shaker and muddle together for 30 seconds so the tequila and pineapple juice mix and the rosemary sprigs infuse with them.
Add the grapefruit juice and fill the shaker with as much ice as you can fit in before shaking.
Strain the grapefruit rosemary cocktail with tequila into a lowball glass with a couple of ice cubes in it. Top it up with soda and slide the rosemary garnish down the side.