Introduction
In all honesty, free / open source data visualization tools in Scala aren't that rich compared to those in other mature data analysis languages, such as R or Python. We might partly attribute this to the lack of rich charting frameworks in Java, and visualization has never been a strong point for big data analytics.
That said, Scala (or more specifically the Hadoop world, including Spark) is catching up with the presence of the Apache incubator project Zeppelin and the highly active Scala bindings (https://github.com/bokeh/bokeh-scala) for the Bokeh project (http://bokeh.pydata.org/en/latest/). With R becoming the first-class citizen in Spark—with the availability of SparkR DataFrames from 1.4 onwards—Spark gets additional visualization from R other than the already existing Python APIs.
As a side note, all existing Java libraries are accessible from Scala. Hence, we are free to borrow any visualization library from Java.