DOC: clean up screenshot section + alt-text

[ci skip]

README.md

@@ -30,13 +30,14 @@ occur. If you have any questions, feel free to contact the development team at
 See the <a href="https://fedarko.github.io/qurro">Qurro website</a> for a list of
 interactive demos using real datasets.
 
-## Screenshot
+### Screenshot: Visualizing KEGG orthologs in metagenomic data from the Red Sea
 
-![Screenshot showing a Qurro visualization of ranked features (which in this dataset correspond to KEGG orthologs) and a box plot of a log ratio of certain features' abundances in samples. Features are selected in the log ratio based on the text they contain; the numerator consists of features with "protein" in their descriptions, and the denominator consists of features with "synthase" in their descriptions. The feature rankings, which were computed by Songbird, are relative to the "Oxygen" sample metadata covariate.](https://github.com/fedarko/qurro/blob/master/screenshots/redsea_data.png)
+![Screenshot showing a Qurro visualization of ranked features (which in this dataset correspond to KEGG orthologs) and a scatterplot plot of the log ratio of certain features' abundances in samples (the log ratio is on the y-axis; the x-axis is set to the temperature associated with the sample, and samples are colorized according to their "Oxygen" field. Features are selected in the log ratio based on their assigned "cluster"; the numerator consists of features in cluster 3, and the denominator consists of features in cluster 8. The feature rankings, which were computed by Songbird, are relative to the "Oxygen" sample metadata field.](https://github.com/fedarko/qurro/blob/master/screenshots/redsea_data.png)
 
-**This visualization (which uses some of the
-[Red Sea metagenome data](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315489/), with ranks generated by
-[songbird](https://github.com/biocore/songbird/)) can be viewed online [here](https://fedarko.github.io/qurro/demos/red_sea/index.html).**
+This visualization (which uses data from
+[this study](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5315489/), with ranks
+generated by [songbird](https://github.com/biocore/songbird/)) can be viewed
+online [here](https://fedarko.github.io/qurro/demos/red_sea/index.html).
 
 ## Installation and Usage