Layered Line Chart, Explained
Layering tidal predictions for each day makes it possible to see hourly trends in the tide across the month. This chart was directly inspired by Adrien Segal's Tidal Datum.
About the data
NOAA makes predicitons for every high tide and low tide for each tide station. The data table has three columns: T, V, and Type. T refers to the predicted time, V to the predicted water level (in feet from bench mark), and Type to the type of tide (high or low). This data comes from NOAA's Tides and Currents Tidal Predictions portal.
| T | V | Type |
|---|---|---|
| 2019-11-08 01:22 | 0.921 | L |
| 2019-11-08 07:53 | 4.668 | H |
| 2019-11-08 14:14 | 1.265 | L |
| 2019-11-08 20:03 | 3.912 | H |
| 2019-11-09 01:54 | 1.132 | L |
| 2019-11-09 08:16 | 4.856 | H |
| 2019-11-09 14:48 | 0.888 | L |
Figure 1: A table showing data from November 8, 2019 and November 9, 2019 showing tidal high/low projections for Moss Landing, Ocean Pier station.
Making the layered line chart
One way to visualize this data is with a line chart. It might be helpful to look at how the line chart was made before reading this section.
The layered line chart is a variation of the line chart. The line chart shows each data point in succession, whereas the layered line chart has a separate line for each day. So first we have to make an x axis that is 24 hours long:
We can add a y axis next. On the y axis, we are going to plot the predicted water level (V from the table). That looks like this:
Now we can add our first line, showing the predicted water level over the course of one day:
Now we can add our second line, showing the predicted water level over the course of the following day:
And we can do this for every day in the dataset. We can also clean up some of the visual clutter, add a line to represent the bench mark sea level for the station, and add some labels:
And voila! The finished chart. Now you can go back to the live layered line chart, or see next chart in the series.