Skip to main content

Things I Learned: Text Alignment in the PyPlot text Method

 I learned how to arrange labels on PyPlot charts yesterday. 

I've mentioned that the scales on the ham radio QSO elevation profiles weren't entirely comparable to one another. Due to the variety off hills, mountain ridges, and coasts the kids and I make POTA activations on, the vertical scales on the elevation chart y axes range from three meters in total to dozens of meters. What's important (and interesting) about the various sites though isn't the sheer number of meters of descent or ascent, but rather, the angle of the terrain with respect to level. That angle gets subtracted from the natural radiation angle of the dipole that houses Project TouCans and, theoretically, extends the distance that TouCans can transmit before bouncing off the F2 layer of the ionosphere.

Consequently, rather than determining a scaling algorithm to make all the elevation profile graphs returned for the various QSOs comparable, I elected to simply calculate the slope of the terrain over the first two wavelengths, (using 2 20 meter wavelengths in these examples since Project TouCans is a 20 meter ham radio.) That just gave me the job of adding a label to each graph (after using scipy.stats to calculate the slope.) Getting the label to the correct spot on the graph took a little more work than I initially expected.

ChatGPT's original suggestion was to use:

plt.text(0.5, 50, slope_st, fontsize=12, ha='center')

That gave back graphs with a slope message that scootched around.

It was centered, but centered on what? Turns out, the text was centered on the coordinates specified in the argument, and those coordinates were literally tied to the data values. So, centering wasn't the easy way for me. I moved to 'left' for the ha argument. Sometimes things looked good:

But also? Sometimes, the label just disappeared:


After a bit of puzzling, I figured out that the data coordinate for 50, specified in the line of code above, for the y coordinate was well off the screen in the plot above.

Ultimately I arrived at the correct—for me—answer. I set ha, the horizontal alignment field, to 'left' rather than center. That caused the text to place it's left-most point at the coordinates specified in the method call: two tenths of the way to the right across the plot. I also used the transform argument:

transform=ax.transAxes

Which made the coordinates relative to the graph's axes, and scaled from 0,0 (lower left corner), to 1,1, (upper right corner.) I had to add a line of code to get the axes object before sending in the transform argument. So I wound up with:

ax = plt.gca()

Followed by 

plt.text(0.2, 0.1, slope_st, fontsize=12, ha='left', transform=ax.transAxes)

Which gave me a label at the same spot on every plot independent of the length of the label or the values on either data axis:






Labels:

Comments

Post a Comment

Please leave your comments on this topic:

Popular posts from this blog

The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though! Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in concept very sim
1 comment
Read more

Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

Now available as a Kindle ebook for 99 cents ! Get a spiffy ebook, and fund more physics The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems , there seem to be fewer explanations of the procedure for deriving the conversion, so here goes! What do we actually want? To convert the Cartesian nabla to the nabla for another coordinate system, say… cylindrical coordinates. What we’ll need: 1. The Cartesian Nabla: 2. A set of equations relating the Cartesian coordinates to cylindrical coordinates: 3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system: How to do it: Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables. The chain
11 comments
Read more

More Cowbell! Record Production using Google Forms and Charts

First, the what : This article shows how to embed a new Google Form into any web page. To demonstrate ths, a chart and form that allow blog readers to control the recording levels of each instrument in Blue Oyster Cult's "(Don't Fear) The Reaper" is used. HTML code from the Google version of the form included on this page is shown and the parts that need to be modified are highlighted. Next, the why : Google recently released an e-mail form feature that allows users of Google Documents to create an e-mail a form that automatically places each user's input into an associated spreadsheet. As it turns out, with a little bit of work, the forms that are created by Google Docs can be embedded into any web page. Now, The Goods: Click on the instrument you want turned up, click the submit button and then refresh the page. Through the magic of Google Forms as soon as you click on submit and refresh this web page, the data chart will update immediately. Turn up the:
Post a Comment
Read more