What was the change in the amount for Performance Share Plan from 2018 to 2019?
{"": {"2019": "\u00a3m", "2018": "\u00a3m"}, "Performance Share Plan": {"2019": 5.1, "2018": 4.7}, "Employee Share Ownership Plan": {"2019": 1.1, "2018": 1.0}, "Total expense recognised in Income Statement": {"2019": 6.2, "2018": 5.7}}
performance_share_plan_2019 = df["Performance Share Plan"]["2019"]
performance_share_plan_2018 = df["Performance Share Plan"]["2018"]
answer = performance_share_plan_2019 - performance_share_plan_2018