Create a pivot table and pivot chart from your data and Group the dates by Month and Year. Sample:
<TABLE style="WIDTH: 394pt; BORDER-COLLAPSE: collapse" cellSpacing=0 cellPadding=0 width=522 border=0 x:str><COLGROUP><COL style="WIDTH: 96pt; mso-width-source: userset; mso-width-alt: 4681" width=128><COL style="WIDTH: 38pt; mso-width-source: userset; mso-width-alt: 1828" width=50><COL style="WIDTH: 68pt; mso-width-source: userset; mso-width-alt: 3291" span=3 width=90><COL style="WIDTH: 56pt; mso-width-source: userset; mso-width-alt: 2706" width=74><TBODY><TR style="HEIGHT: 12.75pt" height=17><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; WIDTH: 96pt; BORDER-BOTTOM: #f0f0f0; HEIGHT: 12.75pt; BACKGROUND-COLOR: transparent" width=128 height=17>Count of Department</TD><TD class=xl23 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; WIDTH: 38pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" width=50> </TD><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; WIDTH: 68pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" width=90>Department</TD><TD class=xl23 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; WIDTH: 68pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" width=90> </TD><TD class=xl23 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; WIDTH: 68pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" width=90> </TD><TD class=xl24 style="BORDER-RIGHT: black 0.5pt solid; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; WIDTH: 56pt; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" width=74> </TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; HEIGHT: 12.75pt; BACKGROUND-COLOR: transparent" height=17>Years</TD><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Date</TD><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Blue</TD><TD class=xl25 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Pink</TD><TD class=xl25 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Yellow</TD><TD class=xl26 style="BORDER-RIGHT: black 0.5pt solid; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Grand Total</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; HEIGHT: 12.75pt; BACKGROUND-COLOR: transparent" height=17>2011</TD><TD class=xl27 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Feb</TD><TD class=xl22 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>4</TD><TD class=xl25 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> </TD><TD class=xl25 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent"> </TD><TD class=xl26 style="BORDER-RIGHT: black 0.5pt solid; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>4</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl34 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; HEIGHT: 12.75pt; BACKGROUND-COLOR: transparent" height=17> </TD><TD class=xl28 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Mar</TD><TD class=xl29 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>15</TD><TD style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>9</TD><TD style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>7</TD><TD class=xl30 style="BORDER-RIGHT: black 0.5pt solid; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>31</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl34 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; HEIGHT: 12.75pt; BACKGROUND-COLOR: transparent" height=17> </TD><TD class=xl28 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent">Apr</TD><TD class=xl29 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>2</TD><TD style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>10</TD><TD style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: #f0f0f0; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>10</TD><TD class=xl30 style="BORDER-RIGHT: black 0.5pt solid; BORDER-TOP: #f0f0f0; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: #f0f0f0; BACKGROUND-COLOR: transparent" align=right x:num>22</TD></TR><TR style="HEIGHT: 12.75pt" height=17><TD class=xl31 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: black 0.5pt solid; HEIGHT: 12.75pt; BACKGROUND-COLOR: transparent" height=17>Grand Total</TD><TD class=xl35 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: black 0.5pt solid; BACKGROUND-COLOR: transparent"> </TD><TD class=xl31 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: black 0.5pt solid; BACKGROUND-COLOR: transparent" align=right x:num>21</TD><TD class=xl32 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: black 0.5pt solid; BACKGROUND-COLOR: transparent" align=right x:num>19</TD><TD class=xl32 style="BORDER-RIGHT: #f0f0f0; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: #f0f0f0; BORDER-BOTTOM: black 0.5pt solid; BACKGROUND-COLOR: transparent" align=right x:num>17</TD><TD class=xl33 style="BORDER-RIGHT: black 0.5pt solid; BORDER-TOP: black 0.5pt solid; BORDER-LEFT: black 0.5pt solid; BORDER-BOTTOM: black 0.5pt solid; BACKGROUND-COLOR: transparent" align=right x:num>57</TD></TR></TBODY></TABLE>