What is the test for abuse of discretion when determining spousal and child support?

We review a trial court's award of child support or spousal support for abuse of discretion. (County of Orange v. Smith (2005) 132 Cal.App.4th 1434, 1445.) In doing so, we keep in mind that the trial court's "`"discretion must be exercised along legal lines, taking into consideration the circumstances of the parties, their necessities and the financial ability of the [supporting spouse.]'" [Citation.].)" (In re Marriage of Rosen (2002) 105 Cal.App.4th 808, 825.) Relevant here, "the former and current statutory guidelines governing the determination of spousal and child support authorize the trial court . . . to consider the earning capacity as well as the actual income of the supporting spouse in determining support, but they do not specify or limit the circumstances under which the trial court may look to earning capacity in addition to, or in place of, actual income in fixing support." (In re Marriage of Simpson (1992) 4 Cal.4th 225, 232. See also Fam. Code, 4330.)