Limit Cargo tags to 5

Author: milliams

Cargo.toml

@@ -8,7 +8,7 @@ license = "MIT"
 edition = "2018"
 repository = "https://github.com/milliams/plotlib"
 categories = ["visualization", "science"]
-keywords = ["plotting", "plot", "graph", "chart", "histogram", "scatter"]
+keywords = ["plotting", "plot", "graph", "chart", "histogram"]
 
 [badges]
 travis-ci = { repository = "milliams/plotlib" }