The phrase "approximately a linear" is correct and usable in written English.
It can be used in contexts where you are describing a relationship or trend that is close to linear but not perfectly so.
Example: "The data shows approximately a linear relationship between temperature and pressure in the experiment."
Alternatives: "roughly linear" or "nearly linear".
The results show that the deflected angle of the damper is approximately a linear function of air velocity inside the burner, while the deflected angle of the jet is approximately a linear function of the deflected angle of the damper.
It is also indicated that the deviation is approximately a linear function of the length of the forecast period.
The residual strength of the fatigue-cracked tubular joints follows approximately a linear relationship with respect to the percentage of the crack area along the brace-to-chord intersection.
At pore bases, the electrolyte concentration generally increases with decreasing bath temperature and increasing current density, being approximately a linear function of true current density at the corresponding surface of charge exchange.
Adelgid populations responded to imidacloprid dosage with approximately a linear relationship between the percent population reduction (probability scale) vs. log of dosage; 50% reduction in populations could be achieved with 0.15 g per 2.5 cm of trunk diameter at breast height (dbh), or 10% of the maximum labeled dosage.
When pressure varies from Pnm to Pmmp, IFT is approximately a linear function of pressure.
As shown, the pressure gradient increases with the rise of flow rate, which approximately keeps a linear relationship.
The expression levels of five miRNAs corresponded approximately to a linear relationship between the glycaemic phenotypes of the three strains.
We find that the proteins are found to follow either an approximately linear or a quadratic dependence on time.
Plotting w as a function of I /(I max − I ) we obtain an approximately linear relationship that also approximately goes through the origin (0, 0) (as it should).
It has been shown that RPE increases in an approximately linear manner as a function of the proportion of an event completed [24] [28], and has scalar characteristics when plotted against percentage of the exercise task completed (time or distance).
