If you are python programmer working on data science, you are certainly very familiar with Matplotlib to visualize your classification or regression result, especially when you are using jupyter notebook. Matplotlib.pyplot is standard tool for data visualization, but there is one problem, it is only for Python.
If you code in Scala, you will need to use alternative, one of the great tools for data visualization with Scala is Vegas.
Vegas works with Jupyter-scala kernel, which is not necessarily tightly integrated with Apache Spark like for example, Spylon-kernel does. jupyter-scala kernel is light weight, great for pure Scala coding without Apache Spark, I have done pure coding with Scala with Jupyter-scala kernel when I do not need to use Apache Spark.
Currently, as I am typing this text, Vegas-viz is only available on Maven repository, up to Scala version 2.11, if you are running Scala 2.12, it is not yet available from Maven repository, meaning you can not download needed jar files from Maven.
The use case I would like to demonstrate is to integrate light weight Jupyter-scala kernel, now it is called Almond
and Apache Spark.
To make Vegas-viz work, you need up to Scala 2.11, you can setup Apache Spark 2.4.4 with Hadoop 2.7 that includes Scala 2.11.
This assume you have Jupyter-notebook already. Then you just follow the install Almond instructions available on Almond website to install Jupyter-scala kernel.
Once installed, you are set to play with Jupyter-scala on Jupyter-notebook and Vegas-viz data visualization tool.
Here is a simple Scala code on Linear Regression from Apache Spark ML library to run under Almond/Jupyter-scala kernel on Jupyter-notebook.
//start with pulling Vegas-viz library jars from Maven repository, your machines needs to connect to the
// internet
import $ivy.`org.vegas-viz::vegas:0.3.11`
import $ivy.`org.vegas-viz::vegas-spark:0.3.11`
//You will see lots of downloads from running the above 2 lines
//Once done, you can import Vegas library for plotting
import vegas._
import vegas.render.WindowRenderer._
//Almond/Jupyter-scala does not integrated with Spark, so you will need to integrate it manually
//by download necessary jars from Maven, for the Spark libraries need to accomplish this demo
import $ivy.`org.jupyter-scala::spark:0.4.2`
import $ivy.`org.apache.spark::spark-sql:2.4.4`
//Again lots of downloads from above 2 lines, once download is done, you are ready to import Spark libs
import org.apache.spark.SparkContext._
import org.apache.spark.rdd._
import org.apache.spark.util.LongAccumulator
import org.apache.log4j._
import org.apache.spark.sql._
//Create Spark session
Logger.getLogger(“org”).setLevel(Level.ERROR)
val spark = SparkSession
.builder
.appName(“Vegas”)
.master(“local[*]”)
.config(“spark.sql.warehouse.dir”, “file:///tmp”)
.getOrCreate()
//and SparkContext sc
val sc = spark.sparkContext
//Even a simple LinearRegression, it is from Spark ML library, and Jupyter-Scala does not have it.
//Then download from Maven for Spark 2.4.4
import $ivy.`org.apache.spark::spark-mllib:2.4.4`
//now import Vegas-viz library
import vegas._
import vegas.data.External._
//Generate random dataset
import scala.math._
var X=Array[Double]()
var Y=Array[Double]()
//Create 100 random pairs of data points in X and Y Array with Double datatype, ranging from 0 to 10
for (i<-0 until 100)
{
var x=random*10
var b=random*10
X:+=x
Y:+=3*x+b
}
//You will notice the dataset is random, how Y=3X+b, it will fit nicely with LinearRegression
//That is not the point, the point is to play with Vegas-viz data visualization
//Create features, label Spark dataframe that sorted by features
import spark.implicits._
val df = sc.parallelize(X zip Y).toDF(“features”,”label”).sort(“features”)
df.show(3,false)
+ — — — — — — — — — -+ — — — — — — — — — +
|features |label |
+ — — — — — — — — — -+ — — — — — — — — — +
|0.12428231703539572|2.6330542403970045|
|0.12875124825893702|9.111634435646987 |
|0.21021408668144725|10.037147132043941|
+ — — — — — — — — — -+ — — — — — — — — — +
only showing top 3 rows
//Import Vegas plotting libraries
import vegas._
import vegas.render.WindowRenderer._
import vegas.sparkExt._
//Plot original Spark dataframe to see how these 100 data points visually
val plot = Vegas(“linear regression”,width=800, height=600).
withDataFrame(df).
mark(Point).
encodeX(“features”, Quantitative).
encodeY(“label”, Quantitative).
mark(Point).
show
//LinearRegression is to find the best fit straight line in the form of
// y_pred=coeffient*X+Intercept
//In simple term, coeffient is weight, intercept is bias
// Below is to call Apache Spark ML LinearRegression API to finish the job
import org.apache.spark.ml.feature.VectorAssembler
//Vectorize feature columns into “feature_new”
val vectorAssembler = new VectorAssembler()
.setInputCols(Array(“features”))
.setOutputCol(“features_new”)
//Create a dataframe that feature column in vector and sorted, that is required by Spark ML API
var vector_df = vectorAssembler.transform(df)
vector_df = vector_df.select(“features_new”, “label”).sort(“features_new”)
vector_df.show(3,false)
/*
output:
+---------------------+------------------+
|features_new |label |
+---------------------+------------------+
|[0.12428231703539572]|2.6330542403970045|
|[0.12875124825893702]|9.111634435646987 |
|[0.21021408668144725]|10.037147132043941|
+---------------------+------------------+
only showing top 3 rows
*/
//Split 100 datapoints to 70 for training and 30 for testing
val splits = vector_df.randomSplit(Array(0.7,0.3))
val train_df = splits(0)
val test_df = splits(1)
print(test_df.count())
//Create LinearRegression model with train_df
import org.apache.spark.ml.regression.LinearRegression
val lr = new LinearRegression()
.setFeaturesCol(“features_new”)
.setLabelCol(“label”)
.setRegParam(0.3)
.setElasticNetParam(0.8)
.setMaxIter(10)
val lr_model = lr.fit(train_df)
// Then test the trained model with test_df
val lr_predictions = lr_model.transform(test_df)
//Then display testing results
lr_predictions.select(“prediction”,”label”,”features_new”).sort(“features_new”).show(3,false)
/*
Output:
+-----------------+------------------+---------------------+
|prediction |label |features_new |
+-----------------+------------------+---------------------+
|6.184323007474771|9.111634435646987 |[0.12875124825893702]|
|6.877166443408621|6.914045632096169 |[0.3713831792261324] |
|8.485151210258538|7.8911343006901555|[0.9344951735265017] |
+-----------------+------------------+---------------------+
only showing top 3 rows
*/
//evaluate the training quality, show R square score
import org.apache.spark.ml.evaluation.RegressionEvaluator
val lr_evaluator = new RegressionEvaluator().setPredictionCol(“prediction”).
setLabelCol(“label”).setMetricName(“r2”)
print(“R Squared (R2) on test data =” + lr_evaluator.evaluate(lr_predictions))
/*
Output:
R Squared (R2) on test data =0.9248598537955983
*/
//Now, based on 100 original data points, to generate 100 predicted value from weight and bias
// that comes from trained model.
// weight is coeffient, which is lr_model.coefficients(0)
// bias is intercept, which is lr_model.intercept
// Best fit linear equation is
// y_pred= weight*x+bias
// which is
//y_pred= lr_model.coefficients(0)*x+ lr_model.intercept
// now generate 100 y_pred from 100 original X based on
// y_pred= lr_model.coefficients(0)*x+ lr_model.intercept
var y_pred=Array[Double]()
for (i<-0 until X.length)
{
y_pred:+=(lr_model.coefficients(0))*X(i)+lr_model.intercept
}
//Create df_pref dataframe, sorted from Array X zip y_pred
val df_pred = sc.parallelize(X zip y_pred).toDF(“features”,”prediction”).sort(“features”)
// Plot it out together with original 100 datapoints
Vegas.layered(“linear regression”,width=800, height=600).
withLayers(
Layer().
withDataFrame(df).
mark(Point).
encodeX(“features”, Quantitative).
encodeY(“label”, Quantitative),
Layer().
withDataFrame(df_pred).
mark(Line).
encodeX(“features”, Quantitative).
encodeY(“prediction”, Quantitative)
).show
With Vegas-viz, you can visualize anything that you see fit from dataset you have on Scala.
As always, code used in this writing is in my github site:
