If you plotted the performance of most major team sport athletes on a Cartesian axis, it would resemble a parabola.The start ambles upward gently as the player’s body matures and becomes accustomed to the rigor of the game’s highest level. Then it takes a sharp, upward spike through a player’s mid-20s until a peak is reached in the late 20s. It is followed by a gradual, then more rapid, descent as the ravages of age and younger competition take their toll.

While most New York Sun readers are hip to this concept, it is only fleetingly grasped in NBA front offices. Often executives look only at a recent peak performance and assume that if the environment is right, that can become the norm, regardless of age. Thus lucrative free agent contracts are handed out to mediocre late prime performers (Derek Fisher’s six year, $37 million deal with the Warriors two summers ago is the best non-Knickerbocker example of this, but most New York fans can simply moan the name of Jerome James and nod solemnly in agreement). The elite teams, however, rarely take these sort of wrongheaded gambles; instead they make their high-risk investments on younger players.

Overall, the Phoenix Suns personnel moves have been a mixed bag, but they have excelled in betting on young experienced players. Last season they took a flyer on forward Boris Diaw as part of their sign and trade deal with Atlanta for Joe Johnson. Diaw hadn’t shown much in his first two seasons in the league and was nearing the end of his rookie pact, but he did have youth in his favor. Phoenix gambled and won big as Diaw averaged 13.3 points, 6.9 rebounds, and 6.3 dimes. He won the Most Improved Player award and helped lead the Suns to a strong season. Marcus Banks, a point guard that the Suns signed to a five-year deal last week, may be this year’s surprise.

Banks hasn’t made many friends in his three-year career. A product of UNLV, he was drafted by Memphis and traded immediately to the Celtics, where he had two speeds, fast and faster. Theoretically that worked well with team president Danny Ainge’s goal of building a young up-tempo team, however Banks’s speed translated into an out of control, turnover prone style that drove coaches nuts. After his rookie season, the Celtics traded him to Los Angeles as part of the Chris Mihm/Gary Payton deal, but the Lakers sent him back to Boston, and the exchange was reworked. Eighteen months later, Boston finally rid themse lves of Banks in the deal that sent Ricky Davis to Minnesota and brought Wally Szcerbiak to the Celts.

By this time, the book on Banks was pretty much ready to go to press. He was a youngster with attributes that never really solidified into useful skills or an NBA-ready game. However, in Minnesota, far off the radar, something clicked. His shooting improved to 47.9% from the 41.3% before the deal. His per 40-minute averages were also impressive: 15.6 points,6.1 assists, and 3.8 rebounds. It looked like the T-Wolves were suddenly set at point guard for the near future. However, Minnesota’s team president, Kevin McHale, showed why he’s the Isiah of the West (without the deep pockets).The T-Wolves drafted Randy Foye, a point guard, and — despite glaring holes elsewhere on the roster — signed Mike James, another point guard to a long-term deal. This made Banks the odd man out.

Phoenix plays at an up-tempo style that should suit Banks’s game well, and his presence gives Steve Nash his first experienced reserve. The two-time MVP has logged more than 5,300 minutes during the past two seasons and looked run down at times in the playoffs. Last season, Diaw functioned as the second point guard, now the 6-foot-8-inch swingman can play more in the frontcourt.

The Banks signing and Amare Stoudemire’s superb play in the Las Vegas summer league have been the good news for Phoenix hoops this off-season, but there have been several missteps. The team dumped both of its draft picks, Kentucky guard Rajon Rondo and Spanish star Sergio Rodriguez, and it continues a policy of being allergic to rookies. They traded Nate Robinson to the Knicks on draft night last year and all but gave the pick that became forward Luol Deng to the Bulls the previous year. The stated reason for these moves are to keep payroll down, but the easiest way to minimize payroll expenses is to stock your roster with players in their rookie contracts. Given the Suns positive experiences with gambling on young veterans, you’d think they would embrace that strategy.

With a payroll hovering near $60 million and extensions due for Diaw and guard Leandro Barbosa, the Suns will near — if not top — the luxury tax threshold. However, cutting off personnel maneuvers in an artificial attempt to avoid penalties is pennywise and pound-foolish. With a healthy Stoudemire, the Suns are one of the favorites to win the title this year. Refusing to spend money on spare parts could be what separates them from having a parade. The Banks signing is a savvy move, and it demonstrates the Suns’ grasp of the performance parabola. But it will have to offset some other blind spots in their personnel strategy in order to be the final piece in the puzzle.

mjohnson@nysun.com