qTeller maintained by Freeling Lab is an excellent resource to take a quick look at some of the public expression datasets available for some plant species. It hosts gene expression datasets for five plant species, namely Arabidopsis, rice, maize, Sorghum, and moss. I use this tool often to take a quick look at the exspression of a gene or a set of genes to confirm or clarify the expression patterns.
Given below in red is the link that you need to drag and drop onto the bookmarks bar.qTeller Data
Once you navigate to the qTeller Visualize Expression page such as this, you can click on the bookmarklet to download the qTeller data as a csv file. The file name will be the species name foloowed by the current gene you have searched for and a csv extension. Which will look like arabidopsis_AT5G14750.csv for an example of Arabidopsis gene. You can search for a new gene and click the bookmarklet and it will download the data for the currently searched gene. I could easily convert this into a plugin, but that would not be universal and would require 3 different plugins for 3 different browsers, and maintaining those will not be worth the time.
Note:- I have checked the bookmarklet in Chrome and Firefox and it works as intended. I will update about Safari as soon as I can access a mac