This year has brought a whole lotta flat. The S&P 500 dipped into negative territory for the year last week (on August 12) and is only up about 1% year to date. The bond market has been flat — the Barclays Aggregate Bond Index has returned just 0.51% so far in 2015. Even the U.S. economy was relatively flat during the first half of 2015, with just 1.5% growth in gross domestic product (GDP) on an annualized basis, well below potential. Flat, flat, and more flat. With the S&P 500 still fairly close to flat on the year (+1.6% on a price basis as of August 14), we look at how likely stocks are to produce a solid year of gains. A look back at history over recent decades is encouraging….