diagrammetern.

/dʌɪəˈɡramɪtə/Etymology: < diagram n. + -meter comb. form2.
1876 §270 Holt's Diagrammeter. This instrument is specially made for measuring the ordinates of indicator-diagrams..and is used much after the manner of a parallel rule.
This entry has not yet been fully updated (first published 1895; most recently modified version published online September 2018).