Fix issue #97: piston demo missing line series feature

Author: 38

examples/piston-demo/Cargo.toml

@@ -6,5 +6,5 @@ edition = "2018"
 
 [dependencies]
 piston_window = "0.105.0"
-plotters = {path = "../..", default_features = false, features = ["piston"]}
+plotters = {path = "../..", default_features = false, features = ["piston", "line_series"]}
 systemstat = "0.1.4"