How to plot a graph with data gaps in Zeppelin?

Question

Dataframe was extracted to a temp table to plot the data density per time unit (1 day):

val dailySummariesDf =
      getDFFromJdbcSource(SparkSession.builder().appName("test").master("local").getOrCreate(), s"SELECT *  FROM values WHERE time > '2020-06-06' and devicename='Voltage' limit 100000000")
        .persist(StorageLevel.MEMORY_ONLY_SER)
    .groupBy($"digital_twin_id", window($"time", "1 day")).count().as("count")    
    .withColumn("windowstart", col("window.start"))
    .withColumn("windowstartlong", unix_timestamp(col("window.start")))
    .orderBy("windowstart")    


 dailySummariesDf. 
 registerTempTable("bank")

Then I plot it with %sql processor

%sql
select   windowstart, count
from bank

and

%sql
select   windowstartlong, count
from bank

What I get is shown below:

So, my expectation is to have gaps in this graph, as there were days with no data at all. But instead I see it being plotted densely, with October days plotted right after August, not showing a gap for September.

How can I force those graphs to display gaps and regard the real X axis values?

Answer 1

Indeed, grouping a dataset by window column won't produce any rows for the intervals that did not contain any original rows within those intervals.

One way to deal with that I can think of, is to add a bunch of fake rows ("manually fill in the gaps" in raw dataset), and only then apply a groupBy/window. For your case, that can be done by creating a trivial one-column dataset containing all the dates within a range you're interested in, and then joining it to your original dataset.

Here is my quick attempt:

import spark.implicits._
import org.apache.spark.sql.types._

// Define sample data
val df = Seq(("a","2021-12-01"),
  ("b","2021-12-01"),
  ("c","2021-12-01"),
  ("a","2021-12-02"),
  ("b","2021-12-17")
).toDF("c","d").withColumn("d",to_timestamp($"d"))

// Define a dummy dataframe for the range 12/01/2021 - 12/30/2021
import org.joda.time.DateTime
import org.joda.time.format.DateTimeFormat
val start = DateTime.parse("2021-12-01",DateTimeFormat.forPattern("yyyy-MM-dd")).getMillis/1000
val end = start + 30*24*60*60
val temp = spark.range(start,end,24*60*60).toDF().withColumn("tc",to_timestamp($"id".cast(TimestampType))).drop($"id")

// Fill the gaps in original dataframe
val nogaps = temp.join(df, temp.col("tc") === df.col("d"), "left")

// Aggregate counts by a tumbling 1-day window
val result = nogaps.groupBy(window($"tc","1 day","1 day","5 hours")).agg(sum(when($"c".isNotNull,1).otherwise(0)).as("count"))
result.withColumn("windowstart",to_date(col("window.start"))).select("windowstart","count").orderBy("windowstart").show(false)
+-----------+-----+                                                             
|windowstart|count|
+-----------+-----+
|2021-12-01 |3    |
|2021-12-02 |1    |
|2021-12-03 |0    |
|2021-12-04 |0    |
|2021-12-05 |0    |
|2021-12-06 |0    |
|2021-12-07 |0    |
|2021-12-08 |0    |
|2021-12-09 |0    |
|2021-12-10 |0    |
|2021-12-11 |0    |
|2021-12-12 |0    |
|2021-12-13 |0    |
|2021-12-14 |0    |
|2021-12-15 |0    |
|2021-12-16 |0    |
|2021-12-17 |1    |
|2021-12-18 |0    |
|2021-12-19 |0    |
|2021-12-20 |0    |
+-----------+-----+

For illustration purposes only :)