Trendlines: forcing the trendline to go through the origin

[Lee Penn]

For some scientific applications (e.g., use of Beer-Lambert Law to determine the concentration of a colored analyte in solution), there is a need to force a trendline to go through the origin.  It seems like I cannot do this via google sheets at this time. Can this change? I like using google apps in my classroom.  

Thank you,

Lee (chemistry nerd)