performance charts: improve chart readabiltiy, add grids, fix x-axis ticks
performance charts: improve chart readabiltiy, add grids, fix x-axis ticks

paulmartel/voltdb,kumarrus/voltdb,deerwalk/voltdb,flybird119/voltdb,creative-quant/voltdb,paulmartel/voltdb,ingted/voltdb,migue/voltdb,wolffcm/voltdb,paulmartel/voltdb,migue/voltdb,zuowang/voltdb,wolffcm/voltdb,wolffcm/voltdb,ingted/voltdb,migue/voltdb,creative-quant/voltdb,wolffcm/voltdb,VoltDB/voltdb,simonzhangsm/voltdb,simonzhangsm/voltdb,paulmartel/voltdb,kumarrus/voltdb,wolffcm/voltdb,deerwalk/voltdb,zuowang/voltdb,zuowang/voltdb,wolffcm/voltdb,migue/voltdb,zuowang/voltdb,migue/voltdb,deerwalk/voltdb,deerwalk/voltdb,creative-quant/voltdb,VoltDB/voltdb,simonzhangsm/voltdb,kumarrus/voltdb,creative-quant/voltdb,creative-quant/voltdb,ingted/voltdb,ingted/voltdb,VoltDB/voltdb,deerwalk/voltdb,flybird119/voltdb,simonzhangsm/voltdb,deerwalk/voltdb,VoltDB/voltdb,zuowang/voltdb,flybird119/voltdb,paulmartel/voltdb,kumarrus/voltdb,kumarrus/voltdb,ingted/voltdb,migue/voltdb,ingted/voltdb,migue/voltdb,kumarrus/voltdb,flybird119/voltdb,zuowang/voltdb,deerwalk/voltdb,flybird119/voltdb,flybird119/voltdb,ingted/voltdb,creative-quant/voltdb,paulmartel/voltdb,zuowang/voltdb,kumarrus/voltdb,migue/voltdb,ingted/voltdb,wolffcm/voltdb,flybird119/voltdb,VoltDB/voltdb,simonzhangsm/voltdb,simonzhangsm/voltdb,zuowang/voltdb,VoltDB/voltdb,VoltDB/voltdb,creative-quant/voltdb,flybird119/voltdb,creative-quant/voltdb,deerwalk/voltdb,simonzhangsm/voltdb,wolffcm/voltdb,simonzhangsm/voltdb,paulmartel/voltdb,paulmartel/voltdb,kumarrus/voltdb