Since getting my meter online, it has been sending its readings to a server in the sky so charts like these can be produced. To really play with the data, I needed to start logging it locally and producing my own graphs. Here is a rambling rundown of how I got from XML being spat out by the device every 6 seconds to something like this:

Step One - Parsing the data

This was the easy part; AndySC had already put together a perl script for reading the serial port and doing the necessary parsing.

Step Two - Publishing the data

Again, not much for me to do here as Andy’s script already publishes the data to a set of topics over MQTT.

Step Three - Logging the data

Finally, something for me to do. A couple years ago, I would have joined Andy’s perl script with one of my own, but python is more my thing these days. I already had a piece of python that subscribes to the appropriate topics and posts the temperature values to twitter. It didn’t take much to get the same script to subscribe to the power data and dump it into a MySQL database on the local machine.

+----------------+---------+
| when           | power |
+----------------+---------+
| 20080502231747 |    0.34 |
| 20080502231753 |    0.33 |
| 20080502231804 |    0.34 |
+----------------+---------+

The table in the database is a very simple one at the moment; logging the power along with a timestamp. An entry is added to the table for every reading from the meter and having been running for 3 days there are just over 11,000 of them. I’ll have to keep an eye on this to make sure it doesn’t run away with my free disk space.

Step Four - Graphing the data

Roo showed me some stuff he’s been playing with using the Google Chart API. Whilst I generally prefer to roll my own (aka, reinvent the wheel), I couldn’t ignore just how easy it is to produce pretty graphs this way.

Before delving into the API, I needed to decide just what I wanted to produce. As there is such a range of chart types available, there are plenty of interesting things that could be done. Initially, however, I decided to stick with the traditional “power-usage-in-the-last-24-hours” chart.

The API has a limit on the amount of data that can be passed to it. So I needed to find a meaningful way to reduce the 4200 data points generated in 24 hours to around 100 at most. I soon settled on using the average value for each 10 minute period. This loses some resolution in the data, but it still shows the trends.

Generating the averages is a simple question of the right query on the database. With some trial and error, I eventually got to:

select concat(substring(substring(`when`,1,11),9,4),'0'), truncate(avg(`power`),3) from currentcost where `when` > SUBDATE(NOW(), INTERVAL 1 DAY) group by substring(`when`,1,11) order by `when`;

Simple huh?

This produces results like this:

+------+-------+
| 2240 | 0.714 |
| 2250 | 0.637 |
| 2300 | 0.406 |
+------+-------+

With that in place, all it took was to throw it together into an appropriate URL for google to generate the image:

http://chart.apis.google.com/chart?chg=100,20,1,7&chxt=y,x&
chxl=0:|0|1|2|3|4|5| 1:|2100|2300|0100|0300|0500|0700|0900|1100|1300|1500|1700|1900& chxp=1,2,11,19,27,36,44,53,61,69,78,86,95&chs=400×200 &cht=lc&chds=0,5&chm=B,f3f3f3,0,0,0&chco=aaaaff& chd=t:0.348,0.348,0.319,0.311,1.285,0.683,0.338,0.349, 0.398,0.39,0.356,0.477,0.274,0.264,0.572,0.637,0.406, 0.382,0.368,0.324,0.325,0.917,0.322,0.358,0.342,0.318, 0.191,0.124,0.209,0.204,0.192,0.213,0.205,0.196,0.374, 1.753,1.537,0.976,0.552,0.556,0.513,0.433,0.363,0.341, 0.575,0.46,0.188,0.123,0.248,0.204,0.195,0.116,0.215, 0.207,0.226,1.574,0.636,0.214,0.206,0.209,0.204,0.648, 0.694,0.628,0.599,0.689,0.743,1.222,0.278,0.394,0.326, 0.394,0.225,0.184,0.13,0.145,0.23,0.226,0.222,0.133, 0.133,0.237,0.231,0.223,0.13,0.234,0.229,0.222,0.203, 0.197,0.193,0.17,0.198,0.176,0.275,0.328,0.287,0.212, 0.159,0.192,0.19,0.212,0.292,0.298,0.418,0.332,0.789,0.894,0.719

Again, simple huh? Well, maybe not so much. Here’s a break down of what that does (and just to confuse matters, this is in a different order to where things appear in the url above…)

http://chart.apis.google.com/chart?

This is base url of the Google Charts api - all the magic comes from here.

chs=400×200

Set the size of the image.

&cht=lc

Set the type of chart - a line chart.

&chg=100,20,1,7

Gives the chart a grid in the background.

&chxt=y,x

&chxl=0:|0|1|2|3|4|5| 1:|2100|2300|0100|0300|0500|0700|0900|1100|1300|1500|1700|1900

&chxp=1,2,11,19,27,36,44,53,61,69,78,86,95

Describes the axis labels. 0 to 5 on the y-axis, and the relevant times along the x-axis. The script figures out approximate positions along the axis for the labels.

&chds=0,5

Specifies the minimum and maximum values for the data - although I occasionally go over 5Kw, I decided it wasn’t worth squeezing the data for 99% of the time when it is below that.

&chm=B,f3f3f3,0,0,0

Fills the area under the line with a light gray.

&chco=aaaaff

Draws the line with a light blue.

&chd=t:0.348,0.348,0.319,0.311,1.285,0.683,0.338,0.349, 0.398,0.39,0.356,0.477,0.274,0.264,0.572,0.637,0.406, 0.382,0.368,0.324,0.325,0.917,0.322,0.358,0.342,0.318, 0.191,0.124,0.209,0.204,0.192,0.213,0.205,0.196,0.374, 1.753,1.537,0.976,0.552,0.556,0.513,0.433,0.363,0.341, 0.575,0.46,0.188,0.123,0.248,0.204,0.195,0.116,0.215, 0.207,0.226,1.574,0.636,0.214,0.206,0.209,0.204,0.648, 0.694,0.628,0.599,0.689,0.743,1.222,0.278,0.394,0.326, 0.394,0.225,0.184,0.13,0.145,0.23,0.226,0.222,0.133, 0.133,0.237,0.231,0.223,0.13,0.234,0.229,0.222,0.203, 0.197,0.193,0.17,0.198,0.176,0.275,0.328,0.287,0.212, 0.159,0.192,0.19,0.212,0.292,0.298,0.418,0.332,0.789,0.894,0.719

Specifies the data points.

What next?

• Chris has started doing some interesting data analysis to see if he can automatically spot ‘events’ on the graph. Will be interesting to see what can be achieved here.
• Andy’s twittering house got some linkage last week from both Earth2Tech and Wired Science. They mention the power orb that was written about last year. I really like the idea of an ambient device for displaying this information - another project for the arduino list.

Tags: currentcost, google charts, graph, home automation

This entry was posted on Monday, May 5th, 2008 at 11:22 pm and is filed under articles, tech. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.