Where we get our news from and what biases those sources have:

The data on the bias in our major sources come from three sources: Media Bias Fact Check, AllSides and Pew Research.

Liberal bias is indicated by negative values, with -10 being the most liberal possible. Conservative bias is indicated by positive values,  with +10 being the most conservative possible. Scores close to or near zero are most Centered in their reporting. Not all news sources were available from every bias source.

All Sides gives a Left, Leans Left, Center, Leans Right, Right ranking, so those values are very granular. The values were recorded along with the confidence in their scoring from the site (not shown). Those rankings was then converted to a -10 to 10 scale (to yield the same size values as the sources) with left and right counting as +/- 10, leans left and leans right +/- 5 and center as 0.

Media Bias Fact Check gives their score on an arrow diagram. We had to measure the size of the arrow (598 pixels [or 619 pixels for certain news sources, which were then scaled to 598 pixels]) and then measure the distance from the left of the arrow in pixels. This was centered and scaled (by multiplying by 2 and dividing by 598 and subtracting 1 and then multiplying by 10). The factual reporting ranking is indicative of how accurate and reliable the source is (Methodology) with very high being almost entirely factual and mixed being some combination of hastily reported (The Hill) and/or bias by omission or biased origin sources (Fox News). 

Pew Research polled American people and asked which sources of news they used and also determined their political leaning biased on a 10 question test. We used that as a proxy for whether or not a source leaned in a certain direction. Thus the values really measure the average partisanship of the viewer and not the actual source. We assumed the viewers are prone to watch news sources that support their beliefs (confirmation bias) which we believe is well enough documented to make that a reasonable assumption. Since the average American scored a left-leaning -1.03 on the Pew results, the corrected values subtract that to re-center the biases around 0.