How can I Influence the number of digits when printing a polynomial expression or when I want to make it a String?

Question

I have an expession (it is the result of Fit) but I want to print or display an abbreviated form with all numbers in it rounded to say 6 decimal digits.

Print[N[polynomial, 6]] does not what I want and ToString also has no option to limit the number of digits.

I rather get an expression with 0.0008834432170369743*d in it (the other coefficients also have physically meaningless digits which I would like to get rid of).

How can I transform an expression to a string limiting all numerical constants in it to some given number of relevant digits?

I further want to use such a string for fit in a constuct like this:

    Show[{ Graphics[myPlot]
         , Graphics[{Black
                     , Text[fit
                           , {2, 1}
                           , {-1, 0}
                           ]
                     }
                    ]
          }
         ]

Answer 1

expr = 0.0008834432170369743*d;

expr /. x_?NumericQ :> Round[x, 10.^-6]

(*  0.000883 d  *)

However, if the the expression contains exact constants that you wish to retain, compare

expr = 0.0008834432170369743*E^(-1.234567890123456789 x) Sin[Pi x/2];

expr /. x_?NumericQ :> Round[x, 10.^-6]

(*  0.000883 2.71828^(-1.23457 x) Sin[1.5708 x]  *)

expr /. x_Real :> Round[x, 10.^-6]

(*  0.000883 E^(-1.23457 x) Sin[(π x)/2]  *)

Answer 2

Perhaps something like the following:

Text[Row[{"fit = ", RandomReal[1, 3].{1, d, d^2}}], 
FormatType -> StandardForm, BaseStyle -> {PrintPrecision -> 2}]

Row addresses your need to augment the output with "fit = ". Using StandardForm (instead of the default TraditionalForm) changes polynomial ordering. The option PrintPrecision controls how much precision is displayed